The headlights only work on Bright. The dim

Could be dimmer switch,may heat up and kill lights or maybe headlamp switch, your at a point were you are going to have to replace parts,first thing is headlamp switch, take apart but leave it pulged in when lights dont work install new switch and see if that works, this is not a diffuclt problem dont know why dealer could not pin point,anyway start there, good luck.

If the low beam head light relay is fine,and the dimmer switch checks out fine,then look at the out put wire from the dimmer switch to the head light relay,check for voltage,then check the relay to see if it energizes.Then follow the wire(out put)from the relay to the head light.Now if power from the dimmer is getting to the relay,but the relay is not energizing,look for a bad relay,or bad relay ground wire.