EDIT 8/19/04 - see page 5 for an updated version of the article

This is only a start. It's not perfect, PLEASE correct any errors you find. Please feel free to add to it because I know I have left out some stuff. For example, I'm clueless when it comes to the sunshades. I don't know if i'm posting this in the correct forum, but i'm hoping, when this is complete enough, one of the mods or admin can put it up somewhere with other tech articles.

Seat Belts
1990 Front seat belts will mount on your 1979-1989 Fox Body mustang, however 1991-1993 Front seat belts, have different mounting tabs, and will not work on 1979-1989 mustangs without cutting new slots for the mounting tabs. 1979-1983 rear black seat belts will work on all 1979-1989 mustangs that are doing a black interior conversion. 1990-1993 mustangs have rear shoulder harness seat belts, and will not directly bolt into a 1979-1989 mustang. If 1990-1993 rear seat belts are used on 79-89 foxes, then the 90-93 rear seat needs to be used, and the seat mounting brackets will need to be welded onto the floor pan, as well as modifications for mounting the seat belts.
For 83-89 mustangs, you can use 79-83 black front and rear seat belts, or 1990 front seat belts with 79-83 rear black belts. All the black buckles are the same for 79-83 and 90-93.
If you plan on using 1990 Front seat belt retractors with a 79-89 Mustang, the 90 seat belt retractors don't have that cable that plugs into the door jam. If you use 1990 front seat belts, you will have a hole in your door jam. To fix this, you can get a rubber plug to plug up the hole. There is an oval rubber plug used on all 79-93 Mustangs under the rear bumper cover where the rear quarter panel meets the tail light panel. I've heard 79 and 80 mustangs have this plug in the door jam and that you can use this plug from those two year mustangs.

Rear ¼ Trim Panels
In 1990, ford mounted the front seat belts about an inch or so higher up. So 1990-1993 rear 1/4 trim panels will not work on a 1979-1989 mustang, without relocating the seat belt mount.
If your converting your 1979-1982 mustang to black interior, then you must use 1979-1982 rear ¼ trim panels. If you have a 1983-1986 mustang, for a direct fit without modifications, you can only use 1983 rear ¼ trim panels. 1983 ¼ trim panels are the same design as the 84-86 panels. 79-82 panels are a different design, and will not directly swap over to 83-86 mustangs. 1983 is the last year for black interior, so if your converting your 83-86 mustang to black interior, good luck locating the ¼ trim panels. If your converting your 87-89 mustang to black interior, then you have two options for ¼ trim panels, if you plan on keeping your 87-89 style interior. You can relocate the front seat belt mount hole and use 90-93 ¼ trim panels, but you’ll also need to convert your rear seat belts to the 90-93 rear shoulder harness belts as well, since 90-93 panels have cut outs for shoulder harness belts. Your other option is to have your 87-89 style panels, dyed black.

Dash
1979-1983 black dashes will swap over to 1984-86 Mustangs, however the dash frame on 79-82 is different. You’ll need to swap your 84-86 dash under frame onto the 79-82 dash, to get everything to bolt up. I believe 1983 dash frames will work on 84-86 mustangs. Do note that the radio opening is different then 84-86 dash radio openings. The glove box latch on 79-83 dashes is different, as well as the glove box. So when using a 79-83 dash, you’ll need to use your 84-86 glove box latch and glove box. 79-83 glove boxes are to large and will not work with your 84-86 mustang. They interfere with the heater core box. Your only option here is to dye your 84-86 glove box black. I know a lot of people are picky about having to dye their interior, but if done right, it will look good. Ford actually painted/dyed a lot their interior. If you look at your 79-86 dash, you’ll see overspray underneath the gauge cluster. You can actually paint/dye your 84-86 dash, if you don’t want to go through the trouble of locating and installing a 79-83 dash. If you use a 79-83 black dash, you may need to drill holes in the dash for where the fuel door/trunk release buttons go.

T-Top Specific
1983 T-Top trim panels(not the End Caps) that cover the T-Top latches, will not work on 1984-1986 Mustangs. The 1983 latch trim panels are a different size, and wont work. Dying your 84-86 T-Top latch panels black is your only option here. The 1983 black end caps will work on your 84-86 mustang, 1979-1982 end caps will work on 83-86 mustangs, but you will need to trim the end caps. The front windshield pillar trim can be had off of a 1979-1983 Mustang hard top or T-Top. It’s all the same, however for T-Tops, ford actually cut the hard top pillar trim panels, so that they wont interfere with the T-Top end cap. If you use hard top windshield Pillar trim pieces, you’ll need to cut the pillar trim pieces where it interferes with the T-Top end cap. This isn’t a big deal, as this is what Ford did as well. Also note that Ford cut the rear ¼ trim panels as well so that they wouldn’t interfere with the rear T-Top end caps. Hard top ¼ trim panels will interfere with the rear T-Top end cap, so you’ll have to cut off the part of the ¼ trim panel that interferes. If your lucky enough to find any of these pieces from a T-Top specific car, then you’ll see how Ford cut these pieces for you, and you wont have to do any cutting.

Convertible Specific
You don’t have too many options here. 1983 was the first year for convertibles, and the last year for black interior. 1983 verts did not have rear power windows. So if you are lucky enough to find black 1983 rear vert ¼ trim panels for your 84-86 convertible, then you’ll either need to convert to manual windows, or modify the panels to mount a power window switch, if your vert is equipped with rear power windows. 1990-1993 rear ¼ trim panels will not work on 83-86 convertibles, unless you convert to 1990-1993 front seat belts, and convert your power window harness to work with the 90-93 rear power window switch. If you can’t find 1983 rear ¼ trim panels for your 84-86 vert, then your only other option is to dye the panels that you already have.

Kick panels and door sills
I believe 90-93 kick panels would be a direct swap for 84-98 mustangs. The 79-83 black kick panels are slightly different. The screw hole that mounts the kick panels to the carpet, is located in a different spot.
Convertible kick panels are different then hatch/coupe kick panels. Door sills from 79-93 are all the same.

Headliner
Coupe header is different then hatchback headliner. The two are not interchangeable, so don't try. 90-92 coupe headliner will work in 79-92 coupes. In 1993, ford changed the dome light, so if you use 93 headliner, you'll have to upgrade your dome light. The same is true for hatchback headliner.

Seats
79-83 hatchback rear black seats wont work with 84-86 hatchbacks. 79-83 is not split folding like how 84-86 is. 90-93 rear seats wont work on 79-89 hatchbacks or coupes unless you upgrade your rear seat mounts to 90-93 rear seat mounts. 79-83 rear coupe black seats will work on all 79-89 coupes. I have 79-83 black rear coupe seats in my 88 coupe, everything fits great and looks OEM.

Carpet
Coupe and hatch carpet will interchange. You can use 79-83 or 90-93. If you use 79-83, you might have to cut additional holes, but the actual carpet will be the same. Convertible carpet is different then hatch/coupe carpet. There are bigger holes cut in the sill area for the seat belts. In addition, convertible carpet goes up higher into the kick panel area.

Center Console
79-83 Black Center consoles will work with all 79-86 mustangs/capris. However, if your using a 79-83 center console in an 84-86 mustang/capri, you may want to swap your stock clock/warning system into the 79-83 console. 85/86 mustangs have orange clocks mounted in their center consoles. You can remove an 85/86 clock/warning system and swap it into a 79-83 console, to keep everything displayed like how it should be for your year mustang. 90-93 black center consoles will swap into any 87-93 mustang. 90-93 black consoles will need a hole drilled for the fuel door release button.

Door Panels
83-86 Convertible door panels and 79-86 hatch/coupe door panels are different. Convertible door panels have more carpet on their door panels then coupe/hatch door panels. The carpet on a convertible door panel goes a little less then half way up the door panel. I've seen an 83 GT Convertible with black interior that had door panels that look very similar to the 85/86 Mustang GT door panels. IMO, for a convertible, these are the best door panels to get, but are very scarce and will be next to impossible to find. Another option is to use 1983 Mustang GLX Convertible black door panels, or convert to the newer style 90-93 door panels. If you try to use hatchback door panels on your 84-86 Mustang, you'll have to modify the drivers door panel slightly because 84-86 Convertibles had front and rear power windows and a 4 way power window control switch on the driverside door panel, where as hatchbacks only have a 2 way power window switch on the driver side door panel.
The 83 GT hatch door panels do not look the same as 85/86 GT door panels, but the 83 GT door panels do look the same as 84 GT door panels. 83 GLX door panels look closest to stock 85/86 GT door panels, but do have significant differences. IMO the 83 GT and GLX door panels are the best looking door panels for a 79-86 black interior conversion, without having to convert your doors to work with 90-93 door panels. IMO, I do not like the way 79-82 door panels look, but to each their own and they will still work. If your converting a 79-82, then you can stick with 79-82 panels or grab some 83 GT or GLX door panels. If your converting an 83-86 to black interior, i'd recommend sticking with 83 GT or GLX door panels. If your converting an 87-89 to black interior, stick with 90-93 door panels.
90-93 convertible door panels are different then 90-93 coupe/hatch door panels. The inner door felt weatherstripping that runs along the width of the window, is longer on convertibles then on hatch/coupes. You can use convertible door panels with an 87-93 hatch/coupe, but the door felt weatherstripping will need to be trimmed.
Do note that some Mustangs did not come with power windows, so if your Mustang/Capri has power windows make sure you get door panels for power windows, and vice versa if you don't have power windows. Also some model mustangs between the years 79-83, had the inside door latch mounted towards the bottom of the door panel, so pay attention to the location of the inside door latch and make sure the door latch on the donor car is located in the same spot as your door latch(the door latch that you pull to open the door).

Cargo Covers - info provided by Foxchassis
The privacy shade/cargo cover breaks down as follows:

'80-early '84 (same length as late '84-'86)
late '84-'86 (same length as '80-early '84)
'87-'89 (shorter than '80-'86 due to redesigned trim)
'90-'93 (shorter than '87-'89 due to redesigned trim to accomodate rear shoulder betls)

'80-early '84 attaches to the underside of the liftgate
late '84-'93 attaches to the rear plastic that surrounds the liftgate striker

I'd like to give credit to those have provided me with info and have corrected me. Thanks to GNVair from corral.net for telling me about the seat belt retractor plugs. Thanks to GT350R for noting the differences between convertible carpet and coupe/hatch carpet, and pointing out the seat belt retractor cable issue, and the info about T-Top end caps, and thanks to Foxchassis for providing the info on the cargo cover differences.