this morning i was driving to work and turned on my rear window defroster(it was cold!) and i kept hearing a relay clicking of and on pretty loud. i was a bit concerned and thought it was the defroster so i turned it off yet i still heard it? it was draining my volts down to 12 everytime it clicked. it was coming from the rear of the car. this sucker was dimming my headlights everytime it clicked. after work it didnt do it. i drove it in the afternoon and still didnt do it. what could it be?