Blowing fuses?

[JAKES 68GMC]

I'm having a problem with my rear taillight/parking/brakelight fuse blowing. When it blows, I still have my parking lights and blinkers, but not brake or taillights, or lights on the inside of the cab.

I was coming home from a friends house and went to turn my lights on. I had headlights, but no inside cablights or rear taillights. So I figured "Ahh, just an old fuse that blew, no big deal". So I changed the fuse the next day, and it worked fine. But the next day, I want to try my lights and the fuse blew again, immediatly. So I stuck another fuse in, same thing. So my dad and I looked some of the wiring over, and looked for bare wires, and we couldn't find anything.

So we took it to a garage and had someone smart work on it. He fixed it. He said "I just shook the hell outta the wiring in the cab, and put another fuse in, and its fine." So I took it, this was last Wednesday. I drove it a few times since with the lights on, and it was fine. But, I went to try my lights today, and the fuse blew immediaetly. I tried shaking the wiring harness, but that didn't help any.

Why does the fuse keep blowing on me, any ideas? Thanks for the help guys, this is driving me nuts.