I have a problem truck. 1980 Chev pick up with tail light and brake light problems. The truck blows the running lights fuse and when it does work the left brake light and tail light go out if brakes are pressed. Tail lights work alone, brake lights work alone, turn signals work alone.
Suggestions for troubleshooting?

Check the stoplight switch mounted directly under the brake pedal arm.
It might be worn and have too much play in it. It might be creating the short
Within it's housing. Check the bulbs that they are seated and making a
Good connection.

Thanks Vandy-1 I found the brake light switch out of the bracket and also a bad ground in the left tail light assembly. Now everything works.