Replace the bulb. This has been a somewhat common thing for Contours. It drove me nuts while my car was under warranty since it would not act up for the technicians. I finally replaced the bulb myself and the problem was gone. About a year ago it happened again, with the other rear bulb.
Most likely the bulb will solve your problem. If it does, you don't need to go down that uncertain path of "what could it be?"
