I have an 82 audiovox head unit MODEL NO. FO-DLX that was recently upgraded with a BT module. The car also has premium sound with the factory amp still installed and working. Has anyone hooked up these older HU to a modern aftermarket amp? My concern is the lack of (audio ground) outputs coming from the head unit itself, there are only four (+) speaker outputs, and I can’t seem to find a straight answer after making a few phone calls to some radio supply stores with no luck. I really don’t want to get rid of the radio being that its got the right looks vs the new retro 2 knob radios, it has BT and an awesome 8 TRACK setup! Any and all help is greatly appreciated.

Questionn being
How would one add the audio ground signals from the radio to new amp four each speaker channel?!

Thank you
Dillon