An update... Today I removed everything from the car and decided it would be much easier to get this working by mocking it up on a bench. The good news is, I haven't harmed the amp. Using an early 90s Mopar radio, (with individual +/- outputs) I hooked up through the high-level inputs and everything worked as it should.
I've learned this 70s system is called a "floating ground", and that modern amps and radios use a "common ground". There exists a $15 item to turn common-ground outputs to floating-ground... The purpose being to save re-wiring your speakers (which isn't that hard in a car, but I guess people use them for old RVs and Buses, so that could be a PIA.)
https://www.scosche.com/floating-ground-adapter...
Essentially what I need is this, but in reverse... Adding the new - feeds rather than removing them. I'm searching for a schematic online and haven't come up with much more than an Amazon review that complains the FGA is just a bunch of capacitors. I'll continue looking into this. I might have to build some sort of isolator myself.
Of course those of you with more knowledge than I are more than welcome to submit a schematic of your own....
There must be people besides myself who appreciate interfacing with old-tech (mechanical pushbuttons, early vacuum florescent displays, integrated CBs, etc.) but would like to have a bit more power, the ability to transmit via Bluetooth, and isolation/protection of the original head unit.