Wiring trouble on 1994 Chevy 1 ton

Help!!!  I have no where else to ask...so here it goes!  I have been banging my head against my truck for 2 days.    I have a 1994 Chevy 1 ton.  Well I am having trouble with the wiring for the rear lights.  There is a main harness that runs to the back of the truck and then another harness plugs in and goes to my various other lights.  I get a fast signal flash on both sides (used to be just 1 now it is both).  yet all my lights are good, nrw bulbs front and back.  I have a 4 tail light system (deck).  While testing, i see that when i run my park lights, what should be a ground going to my tails, actually turns +ive when tested.  I have good park lights across, but my signals are just a fast flash.  Any ideas???? HELP!

Thanks!

Mitch