Hoist
You may have temporarily solved your problem when you pulled on your headunit. Sometimes with all of the wiring bunched up in the back of the head unit, your RCAs can get pulled in every direction. Inside most head units, the RCAs are connected by a small plug that is connected into the PCB board. If pulled on enough, 1/2 of the plug will pull away from the pins and not make a connection on one of the channels. If the problem persists later down the road, open your headunit, follow the RCAs to the board, and make sure that the harness is completly plugged in.