Pulling the amp remote lines off didn't do anything. Disconnecting the black ground wire, and reconnecting it to bare metal in the car, however, did. Digging around in the dash a bit more, I found two more blue/black wire pairs that had been clipped. The blue wires carried +12V, and the black were 0V, so I tested the connection with one of the black wires, which also worked.

I'll still be checking the amp remote wires, to see if they really are amp remote, or if they're attempting to power the amps, or something silly like that.

Thanks for the suggestions. Hopefully this'll let me finish up this weekend.