I have a 1979 F100 that I recently purchased. The tail lights aren’t working. I have narrowed it down considerably already. I can unplug the connection behind the rear bumper and it shows 12 volts going to the tail lights, but it isn’t strong enough to power a 12 volt bulb. I changed the light switch in the truck. Also, when I first got the truck the tail lights weren’t working but driving it home they began to work. They stopped working a couple of days later. The brake lights work fine and the front parking lights work as well. Any ideas? Thanks in advance.

 

maybe check the connection at the headlight switch, maybe replace the headlight switch

 

Ok thank you. I replaced the headlight switch and there was no change. I guess it’s down to checking that wire to find a bad spot.

 

The brake (stop) lamp switch fits on the brake pedal and it's adjustable.

 

If you tested for 12 volts at rear of truck, was it with a DVOM?? If so with parking lights on test the voltage at the battery. Then go back to rear of truck where you disconnected the connector and check your voltage there. Your DVOM negative lead needs to be hooked to the frame, NOT the body. Sand a bit of the frame clean where you hook the Neg lead. If no voltage loss, great. If 1 0r 2 TENHT'S of a volt less than battery voltage you can assume all is well to that point. If a greater voltage loss, start at the thru fire wall connector under hood. There are two, one with more wires for all in engine compartment. The other for the lighting rear of truck. Unplug that connector, inspect for corrosion. Test voltage here, parking lights still on. If no great voltage loss, then problem seems in wiring along the frame. If great loss at connector, problem under the dash. Assuming your verified both bulbs are good. One more thought, look and see at fuses if there is one for tail lights or marker. The fuse receptacles rust very badly in these older trucks and lose their spring tension.