I have a 1968 coupe with a strange problem. The driver's side rear brake light and turn signal are inoperative when I'm driving. The lights on the rear passenger side, and both front turn signals, function fine. However, if I turn on the headlights or parking lights, the driver's side tail light comes on, and the brake and turn signal on the driver's side starts working correctly! That is, the rear tail light gets brighter when I step on the brakes, and the turn signal works an flashes. Any ideas how to track this down? I have been staring at the wiring diagram but am not sure what to check. Everything else electrical on the car is functional, cigarette lighter, side marker lights, fog lights, headlights and hood mounted turn indicators all work. Thanks!