I had the exact same problem. Well, kinda. I left the roof open and it got rained on. I thought the same thing as you too, but this is not the case.
The fader switch on the dash has two channels (four wires) going into it from radio, two for left and two for right. The fader itself simply makes the front or rear quieter by applying resistance to the circuit. There are 4 hot wires and two ground wires going out of the fader. Therefor since the left and the right side are using a common ground, if the ground wire on whichever side failed, you won't get sound on that side.