Before it broke, did the volume vary depending on the engine RPM? On some cars, the volume automatically lowers when the engine is idling at a stop light and then rises as you accelerate (the acceleration would be expected to give louder engine noise, hence the volume increasing).
If the volume automatically raises and lowers, the wiring is slightly more complicated and will be a little tougher to troubleshoot on your own.
Either way, intermittent problems are the pits. Hopefully, the bad wiring will eventually result in a short circuit, some blown fuses, and possibly some other fried components (hopefully, not too many in the car's ignition circuitry). The damage from the short will be easier to detect than the intermittent problems.
(This sounds silly, but putting up with intermittent problems until a part finally broke down completely was just part of the package in controlling satellites, where the ground system could be complex enough that the maintenance guys just weren't going to be able to pinpoint intermittent problems.)