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Especially if you have a tilt column in your truck! There's so many places where a wire can get pinched or in a bind, or the insulation can get worn away or cracked. It doesn't take much bare wire making contact where it shouldn't to create BIG problems.I would guess some insulation has worn through and semi shorting to ground. The turn signal might "sort of " work until the fuse heats up enough to blow. See if you are getting some continuity to ground with a voltmeter from one side or the other with the fuse pulled out. Try this with the turn signal engaged and disengaged. If you find some ground where it shouldn't be (likely) you will need to trace the wire and find the bare spot. Good luck and let us know what you find.