1974 GMC Sprint. With headlights off, brake lights work. With headlights on, one brake light doesn't work. One side does. Tail lights, reverse lights and turn signals all work on both sides.

Do your brakes lights work? You replaced the switch was this the light switch that is found on the firewall that is activated when the brakes are applied?
If not, replace that switch. it is found underneath the dash on the driver side.It has a button that is depressed when the brakes are applied.

brake lights not working - 1997 GMC Jimmy - RepairPal

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Jan 11, 2011 - hi about 2 days ago i realized my brake lights were not working but my turn signal lights were working and parking lights as well but no brake ...

Tail Lights don't work but third brake light does, not fuses ...

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Sep 23, 2009 - 97 GMC Jimmy 4x4 blinkers work, reverse lights work, above window ... I just lost hazard lights and rear brake lights, they were working 2 days ... If it is a frayed wire or solder point, then you can repair it, but not any worn contacts. ... Does anyone have any suggestions on 1997 Honda Accord tail & brake lig.

Replace the right rear brake light bulb. It is possible that the filament has burnt out and has fallen across to rest on the brake light filament. This would cause the condition you are experiencing.

It sounds as though you have either a frayed wire between the brake control circuit and tail/running light circuit of the truck side of the plug, or someone wired the truck side of the plug incorrectly.

Think about it. The parking brake lights come on when the controller is activated. More than likely the tail lights are also.

The tail lights are also going to come on when the headlights are on. This would cause the trailer brakes to be activated IF there is a short in the TRUCK side of the plug itself.

That's where I would personally start investigating, The truck side of the trailer plug.

Hope this helps.

reverse lights are on a different circuit to the brake/tail/clearance lights. Check fuses for the others.

pull your brake light assmb out and check your connections including ground

If you have any extra fuses, try replacing the ones for the head lights and tail lights. Make sure the fuses you replace are the same color as the ones you replace them with, that way the voltages are the same. Another issue is that a flasher could be going out.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/erik_3934b541f17c4e99

I had this problem with my 2007 Honda CRV -- all the lights worked EXCEPT the tail/running lights and the license plate light. The other tail lights worked -- brake, reverse, indicators, and hazards. Front headlights also worked, but when the headlights went on the back tail lights did not (not to safe for driving at night!!)
The fuse was #15 in the interior fuse box (under the dash driver side) with the label "small exterior lights" ( also check fuse #19 labeled "small lights main"). The cause of my problem, I believe, is a bad or lose wire in my license plate light.

try replacing the turn signal switch, as the brake/tail lights are all routed thorough it. common problem with most 90's GM cars. this may help, but I've never seen only the third light work. sounds more like wiring has been tapped into incorrectly. good luck

switch burned up due to a short in the tail light wire