Way overboard question but does or did you vehicle ever have a trailer wire harness added?
I have seen what it can do to the lights....
Another question, do all the brake light / turn signals work and are all the same brightness?
I saw a guy used an 1156 bulb where an 1157 dual filament bulb was needed. This caused some big problems.
Totally off subject but fun story, I think. A customer pulled in to the Ford dealership where I was a service advisor with a pick up saying his brake lights keep blowing fuses. I took a look and he did have a trailer harness with a plug in, covered, connector. The cover was open and the socket was packed full of SNOW!!! Good times back in 1973.