I would think that the two harnesses would be simular. the Cat & Bonneville have the tail lights that go around the top of the rear panel and the Grand Prix has the lights in the rear panel, simular to 66 GTO's. They are both a 3 segment Tail lamp ie. 3 tail w/2 stop. I would look at the plugs and make sure that all wire colors are the same. GM didn't make drastic changes to wiring colors between years. Also I would check the turn signal switch. The brake circuit comes from the fuse block to the brake light switch and then to the directional signal switch. From there it will go to the front, Dash indicators, and rear lights. Check your grounds at the rear of the car too. as one light beams it may be using another bulb for its ground circuit resulting in a non lighting situation. Good luck