I'm going to disassemble the left and right tail light assemblies to see if any signs of water. Does water have to get into contact with the tail light bulb connector to cause a short? I'm wondering because brake and hazard lights are in the same light assembly, but they still work.
You are probably on the right track, and that's where I would start, keep in mind that any place where water could bridge the wires can be the cause of the short. Other than the assembly this could be:
- Connectors: modular connectors could be shorted by water. The back of the tail light assembly uses such a connector, but so does the other end of the wire, and possibly somewhere in between.
- Bare wires: if a wire's protective sheeting has been off the bare wires could be shorted
- The fuse board: If the fuse board got wet it could potentially short the tail light circuit. This is extremely unlikely though as if you had moisture there you'd have a lot more going wrong