89 4x4 4.3L PU Blows Tailight Fuses

I have been replacing the tailight fuse on several occasions in the last 6 months or so. Each time I replace, the new fuse blows quicker until just recently when the replacement fuse blew immediately. The problem is linked to use of the parking and headlights - when the lights are on, the fuse blows. When the fuse blows, the interior dome light goes on, all parking lights go off and the courtesy lights go off. My Wife said that twice, it blew when she hit a bump in the road. Any clues? (I ordered a new Headlight/Parking Light/Dome Light/ Dimmer Switch Assy to try.) Sounds like a short but where?