Front signals work but both back signals flash either direction

I have a 92 chevy s-10 traded the bed with same,model type truck once hooked up lights for the rear they work as in brakes but once you turn on headlights they stop working also turn signal for front works but both back ones are flashing as if hazards are on then there are no tail lights what's so ever and brake lights and reverse are both on the same bulb instead of seperate ones took old harness off of old bed and tried it and still same issue brand new bulbs and replaced silver flasher relay under clutch so getting stumped any help would be awesome