same problem for many years for these trucks.. wont turn off headlights or wrong positions on headlight selector not working.... many on forum says to replace headlight switch..however if you pop off the headlight console which comes with the dimmer switch unit, unhook the wire harness at the back of the headlight switch unit. dissassemble the the headlight switch unit and u will find hundreds of brass filings after many years of rubbing on contacts that have shorted the connector plates meaning shorting the function of the headlight switch positions. spray electrical cleaner and brush with good brush. also the tangs on the other unit might contain brass filings. banging my head on this one but finally fixed for me... good luck folks.......