Thanks for your input. I've ohmed out all of the wire connections from the head unit to the amplifier and they all check out.

It seems to me that the problem would more likely be before the amplifier because there is no sound coming from both the right door and right rear speakers. It looks like the head unit outputs 1 left and 1 right signal, and those are split before the amplifier to input 4 channels (left/right door & left/right rear). Either both the right door and right rear channels are blown out, or the amp isn't even getting any right channel signals.

The only way I can think of to confirm if it is a head unit or amplifier problem is to swap the left/right outputs from the head unit to see if the working speakers flip sides. Do you know what the trick is to pull the wires out of the harness so I could swap the Left/Right wires on the head unit harness? I tried pulling on them with a pair of pliers, but they seem locked into the harness.

Thanks again for your help.