had a similar problem before where the tail light circuit is powered up on the application of the brakes . problem faulty tail light with a short between brake and tail sending 12v back into the tail light / dash light circuit. fix on hotrod with custom lights was to install diode to stop feedback easier than fixing custom lights. That's where I would start at the tail lights isolate one at a time to figure which one it is. Tail lights should be the only place where these two cicuits brakes and tail lights are close together

1950 Chevrolet 3100 in Oz