I can't seem to find a bare metal piece in my dash, but i bought a Scosche factory wiring harness and it says one of the wires is the "Chassis Ground (12-)". I've also been experimenting and i found that it will keep playing as long as you don't turn the volume up past 17, which isn't all that loud.

I'm not sure if the original unit was a bose it is just the factory stereo for a 93 trans am that i took out and a clarion is what i just put in.