Blown Fuses

[BudTX]

My son's 1983 K20 keeps blowing the fuse to the tail lights. When I replace the fuse and turn the lights on with the engine off, the tail lights are very dim. If I start the truck and then turn the tail lights on, the fuse blows. It takes a 20 amp blade fuse. I tried a 25 amp (I know, I know), but it still blew.

Any ideas? I know next to nothing about troubleshooting electrical problems (except that you don't put a fuse with a higher rating in when you short one out ).

Thanks,

Bud