1966 F100 Brake Light Problems
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2nd Gear
Joined: Feb 2026
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1966 F100 Brake Light Problems
New owner of a 1966 F100, a basket case but slowly coming around. I’m having an issue with the brake lights and hopefully someone can give me some tips. Brake lights work fine until I turn the headlights on, then they don’t work, tail lights work but not the brake lights. I’m thinking it’s a ground issue, maybe some rust or corrosion behind the tail lights making it act weird? Anyone else dealt with a similar issue? Thanks in advance.
Lead Gopher
Joined: Aug 2012
Posts: 10,065
Likes: 1,615
From: Seattle
Yes, I have dealt with that some myself as well as helped others more than once. If you have a multimeter or ohmmeter you can check the resistance between the taillight bucket and the bed of the truck. Paint, rust, greases and oils, looseness, etc. can block a clean current flow between two parts. I like to check the resistance over a longer area, so I check between the battery negative when it is hooked up and the taillight housing when it is still screwed to the bed. Rather than retype it I'll post a link to a prior thread. Give it a try and if you have any questions come back to this thread you started and ask!
Light issue... - Ford Truck Enthusiasts Forums
The reason over a longer area is a good test is this will check from the taillight housing through the bed and its bolt connections to the frame, then through the frame to the battery negative post through the engine/starter or whatever the battery negative goes to indirectly. The less ohms the better. Zero ohms is the theoretical perfect.
Light issue... - Ford Truck Enthusiasts Forums
The reason over a longer area is a good test is this will check from the taillight housing through the bed and its bolt connections to the frame, then through the frame to the battery negative post through the engine/starter or whatever the battery negative goes to indirectly. The less ohms the better. Zero ohms is the theoretical perfect.
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