Turn signal is knocking out brake light

I have a 1995 F250 and the turn signals are not working. I have checked the fuse, turn signal relay and even replaced the multi-function switch. Hazards work and brake lights work. I have noticed also that when I have the brakes on and then turn on a turn signal, the brake light goes out.

Any suggestions on how to fix this?

I would check on grounding issues: check how the rear lights assemblies are grounded and clean the connections.

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Concur w/advice above, good chance this is a problem with the rear-light fixture to chassis ground connection. An auto tech would measure the voltage going to the turn signal light, probing between that wire and chassis ground. If that was working ok, but the light wasn’t working, then they’d suspect either the bulb was out or the ground problem. It’s pretty common to have loose ground problems in tail-lights. I had one on my Corolla one time, with similar weird symptoms.

I suggest you look for after market trailer light wiring. Often tail light problems are either in the added on wires or caused by those wires.

Also a good idea. I had a license plate light fail recently b/c of faulty trailer wiring on my truck.