From the submitter:
"Well, I have car music again. It doesn't sound great, and I have to manually apply +5VDC to turn on the amp, then disconnect another cable to turn it back off... but it works for now."
From submitter azgli: The speaker system at a friend's party was missing a proprietary 13-wire cable between the main unit and the speaker controller. This was my solution to the problem - 13 individual wires insulated with Scotch tape! More Scotch tape kept them from shorting when the sub started rattling things around.