I don't know what year of truck you have but the dimmer switches can cause big problems. If you were checking for shorts to ground I would be checking the wire that goes from the headlight switch to feed the dimmer switch, with everything still attached. If you start to remove wires you might alter the problem.

1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed

My Chevy Master 4 Door is on the Road!