Probably lost the ground to the rear lamps, so they are feeding back.
No turn signal or brake lights working when headlights are on
Hello, I have a 94 Safari that was working great up until today. Someone following me flipped me off and said something about learning to signal. This prompted me to get out and check my lights. The rear brake lights, including the 3rd brake light were all on which in turn causes my signal lights not to work. I found out after more poking around that if I turn the headlights off everything works great. I get good signals left and right, as well as all 3 of my brake lights working as they should. I've seen some similar posts but nothing quite the same as what I'm experiencing. Any help pointing me in the right direction would be much appreciated. I'm just waiting for the rain to stop before testing the circuit a little further. Figured I'd see if I can get a little insight while I wait it out.
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Makes sense. I did just finish building some cabinets back there on the weekend. I must have knocked something loose without realizing. This van didn't even run when I got it, and 90% of the issues were just corroded or rusted ground connections so this doesn't surprise me. Thanks for the quick response. Hopefully it's that simple.Probably lost the ground to the rear lamps, so they are feeding back.
Look to see if you ran a screw through the wiring harness, also.
Check that all the signal lite/brake lite bulbs are inserted correctly, as they can be inserted wrong and will cause this kind of problem .. Good Luck 
I tried everything I can think of. Checked the harness front to back, tried new bulbs, grounded the rear lights and checked their sockets. My brake lights seem to be working good now but they come on at about 80% brightness with my headlights. Then when I hit the brake pedal they get brighter. Seems normal I think. The 3rd brake light only comes on when the brakes are pushed now. My signal bulbs actually do work now as well but the 2nd filament on it (for the brake lights) is so bright right next to it that you can barely even tell its flashing. With the headlights off everything appears to be working 100%. Any other ideas? I just can't seem to figure out any reason for this to have happened. The closest I got to the wiring when building was some spray foam above the driver wheel well. But the wiring is fine. I pulled out my foam and checked every inch of it from one end to the other.
The "2nd" filament, the dimmer one, are the running lights, commonly called tail lights. The brighter ones ARE the turn signals and brake. The 3rd brake lamp is on a separate circuit.
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