You have a short and they can be tough to find. The brake light switch might be sticking. Check all your lights to see if they are on when they should'nt be. Try to do it in the dark as it might only be glowing a little bit. Dont forget the dome light if you have one. If not a light you might try crawling all under to look for any worn spots. Underneath is a good place to start but you should look under the dash and the hood. Any bare spot that can touch metal even a little bit will cause you trouble. I am not great with wiring but it is probably something simple if you can just look in the right spot. Good luck.

I like old trucks