2000 Mustang GT, Taillights stay on (dim) when headlight switch is Off

If it were my car, the first thing I would check is the grounds in the trunk. Take out one of the bulbs and measure the resistance from the ground terminal back to battery negative (car off).

Next, it would be handy to know if the stray voltage is coming through the stop lights or the parking lights.

Pull the following fuses one at a time and report the results. F2.41, F2.18, F1.10, F2.19. Note, F1 is the Battery junction box, F2 is the central junction box.

This problem could be in the electronic flasher module or the multifunction switch as well as the head light switch. Where to look depends upon where the voltage is coming from.

Sounds like the light switch is bad. I had one go bad on a 1990 Ford Escort GT. The lights would cut off at night while I was driving and then cut back on a few seconds later. Dangerous!

Might have been handy for us to know these are aftermarket LED assembles.

Some cars have problems with LED lights because they draw so little current. This sometimes causes the lamp monitor to "think" bulbs are blown.

The ususal fix is to install a ballast resistor to add addtional load in order to fool the monitoring circuits. Note, the problem usually presents as a fast blink. Some cars will give a tone when the brakes are applied.

I've about had it

I have the exact same setup on a 02 GT and I discovered this problem last night after installing a 03 cobra bumper and fogs. tail lights and 3rd brake light glow very very dim any time the engine is running. This is just the latest problem in tracking this electrical gremlin I've had. The headlight switch was replaced 6 months ago to see if it helped my voltage issues which it didn't. I'm now leaning towards a bad multifunction switch or GEM (generic electronic module?!?) After this light problem started last night my car died when I'd pull the headlight switch to turn it on.

Did you find what the problem was?


--------------different but maybe related?!?---------
I also sometimes open my car door and the car voltage drops down to 9volts. I shut the door and it goes back to 12.4. If this happens I can hit the door lock button on my keyless entry and the problem goes away.