Per 79 EVTM, 1,3,4,6,9,16,17 are unused. Looks like same for 80.
Checked mine long ago seeking new unused circuits. None.
Could tell right away looking into a open fuse position with a light..
No contacts for fuses or no wiring to it at all.
If there were, where did the wires route to?
Add-a-fuse taps work, but have limitation of side clearance from other fuses and fuse box cover lid clearance.
Have one in my truck for powering the added backup alarm with switch. Barely fit but did.