The first thing you need to check is the tailight's ground and for proper wiring to the sockets, and check for ground doesn't mean 'looks okay.'

There are more birds with tailights wired incorrectly than correcly wired ones. Ever hear of people saying that there is a wiring glitch in the tailights, implying it's factory, osmosis, or a short? Yeah, it's a short all right, in the installer's brain.

Poor ground and inverted socket/bulb (or a combination) installation can cause a fasinating list of problems, and the list goes on and on. Many defy logic, being great-great nephews of Uncle Murphy.

I'm not ruling out the switch. The point is that instead of start throwing money at this problem, you need to make 100% sure that there is no ground problem, inverted wire connections and inverted bulbs.