hi! sounds like you have grounding issues. did you swap the complete harness or just the rear chassis part? i put a 71 chevy harness in my 67 gmc and had trouble cause head light plugs were different. do the front marker-blinkers do the same as rear or just rear? i would start from scratch. clean both front-rear bulb sockets.then remove clean grounds. truck should have separate ground wires at tailights coming from stock harness. make sure you have a good body ground to frame. buy new bulbs(i have bought new bulbs that were bad) if nothing changes from before start where the harness comes out of cab. near the throttle cable comes out of firewall is the rear lighting harness plug. it has 4 wires brown,green,lt green and yellow use a test light at the firewall plug. when you touch each of the 4 terminals it should light up according to what the taillights would. for instance the brown wire runs both tailights, when you pull headlight switch out it should light up the test light. green is rh turn siginal. it should make test light blink. and so on. if everything works fine at that plug you know trouble is in rear harness. then go to back and test at each wire and that will tell you if you have a broken wire in harness. sorry to be so long winded but i went through the same trouble, pulling my hair out till i started over and i havent had any trouble since hope this helped /forums/images/graemlins/yikes.gif /forums/images/graemlins/yikes.gif /forums/images/graemlins/yikes.gif