The fuse will blow if there's a short to ground somewhere in the turn signal circuit. It makes the current bypass the switch and lights, and therefore turns the fuse into a load, which it can't handle and it blows. You will have to use a test light or a multimeter and trace the circuit to find the short. Fix that and it will fix the problem of the fuse. If you need tips on tracing the circuit just ask, I've got wiring schematics for your truck so I can help.
Oh and I advise not using a very high amp fuse, since the fuse is meant to cut the power if the current is too high so the wiring doesn't get damaged. Going with a high amp fuse might risk burning up some wires somewhere, especially if its shorted somewhere which it probably is.
... any ideas
