Wire Schematic 68-77 Tail and Stop Lamp Simplified
It’s easy to check the system… and there are different ways to approach this. Since most tail lamp issues are a ground or fuse issue… check the fuse.. There is more detail on this below.
I’d start by removing the tail lamp lens and bulb and test for power at the spring loaded prong inside that the bulb hits. If you don’t have a helper, pull the brake lamp switch connector from the switch and place a jumper wire between the two terminals. When testing for power, you should always use a verified ground from the frame. If the fuse test good, and you don’t have power there then go inside the car.
If you do have power there then test the brass socket for continuity to ground… If you don’t have ground then test the ground wire on the back of the housing and see if it has continuity to the frame. If not, repair the ground… and its possible the brass sockets has corroded and you can remove the housing, press them out, clean them and then press it back in place without replacing the housing.
Inside the car; go to the brake lamp switch first and see if you have power at the switch on the orange wire, then push the pedal and see if you have power coming out on the white. If you don’t the switch is defective or the Lamps fuse is blown. If you check the fuse test for power on the left side of the fuse terminal, not the fuse and then test the right terminal. It’s possible for the fuse to be good and the terminals corroded.
If both are good and you have power on the white wire, then pull the connector off the brake lamp switch and put a jumper wire in place. Then go to the turn signal switch connector and test for power on the white wire (E) This is your brake lamp power. If you don’t have power there then the wire is broke between the switch connector and the brake lamp switch. If you do, then test the turn signal switch connector again at points (F and G). With the switch depressed and the turn signals off you should have power on both these wires! If you don’t the turn signal switch connector is either corroded or the switch is defective.
If you do have power on the two connectors, then test for power at the front to rear connector located behind the left side kick panel. Clean the contacts there and if you have power there test for power on the other side with a test light that can pierce the wires.
The turn signal issue is probably related to the other lamp issue, but the only difference is the turn signals (flashers) get their power from the purple wire. When you turn on the turn signals, the power supply to the side you selected switches from the white wire to the purple (D). When this happens the power is on a flasher which causes that side light to flash…. But as I stated above, the issue is probably in the tail lamp circuit with the only difference for the turn signals being the power feed wire from the flasher.
I hope this all makes sense and if not… post back.