Thanks to the wiring diagram from BobR, I was able to determine that I had no 12vdc to the 2 amplifier power wires. I was unable to determine the cause of the no-power state, so I tapped into the radio wire that's hot all the time and ran the 12vdc to the 2 wires for the amp. This fixed the amp problem, but I still had the old problem of the factory radio having bad or corroded contacts in the volume/balance control. I was only getting sound to the left speakers if I fiddled with the knob. Since I had a factory cd changer, I didn't want to change the radio, so I removed the bottom panel of the radio (only 2 small screws) and sprayed the volume/balance mechanism with electrical contact cleaner, turned the knob rapidly back and forth to clean the contacts, then blew dry with compressed air. The radio has been working great for the past week.