a common reason for lighting fuses to blow, that often drives guys nuts, is a partially blown bulb. Ford uses heavy gauge filaments in their turn signal bulbs and they can burn out but still work. Check all your signal bulbs, particularly the rear ones, and look closely at them in front of something white. If there is any little smoking on the glass, or if the filaments appear rough instead of smooth, they are burnt out and need to be replaced.
The reason this can cause the fuse to pop is that now one end is free to move around, so when the truck gets jostled (like it does during a burnout) the filament can bounce and hit another filament and pop the fuse.
