Hello Buckwild8714 welcome to the group.

When you press the brake pedal and power is sent to the 2 filament stop/marker light bulb an there is no ground present from the bulb socket the power might or will backfeed up the other bulb circuit which would be the marker light. Which might cause other interior lights to turn on.

I think you might be describing a ground issue. Besides the ground connection at the frame what about the ground connection at each bulb? Have you looked inside each bulb socket? Are they clean or are they full of decades old dielectric that has hardedned and become an insulator?

Do you have a power probe or multimeter/DVOM or a simple test light at least? It would be nice to probe the sockets for grounds.

 

Welcome to DC Buckwild8714.

 

Hey so I was also thinking over the years I have seen some funny stuff.

If someone put a single filament bulb in your dual filament tail light socket this would jump the 2 circuits and bring about the same problem as a bad ground with the backfeeding.

In fact more so because with the right bulb and a bad ground the power will be filtered through 2 light elements and that will act as a resistor unto the power. But when a 1156 or 3156 bulb goes into a 1157 or 3157 socket it directly connects the stop and marker light circuits.

I think you should also check all the tail light bulbs. Make sure they are the correct bulb.

 

Turns out it was g401 under the carpet behind the passenger seat. Someone had moved all those grounds to a hole they made with a self tapper and never cleaned the paint off of the steel there. New bolt on fresh steel and clean ring terminals and boom everything works!
thanks guys! Gonna clean all the light sockets next. Just waiting on a new can of Deoxit to arrive.