Now that I understand the problem, I realize how much of a pain it can be. On the list of things to do - 1) Replace the scrounged wiring harness with a fresh one that includes a ground wire. 2) Reroute the RCA cables through the dashboard so they don't get pinched. 3) Ground the empeg's sled.

For #3, I have a question. Would the sled be sufficiently grounded if the back were mounted to the frame of the car using the strip of metal originally meant to mount the CD player?