1995 GMC C1500 Intermittant Starting

The problem I have is that the truck will turn over but intermitantly not start. When this happens if I put the truck in reverse, the rear running lights come on rather than the B/U lights. If I turn on the headlights then the running lights go off, still no B/U lights. Then if I turn on the emergency flashers with the key in OFF position my dash lights go crazy. Sounds like a short to me but I am having a hard time tracking it. After working with it for awhile, it will all of a sudden start right up and run fine. The way I can tell if my truck will start is by looking at the rear lights. When it starts everything works fine.

Help!