'92 Explorer rough idle...only when defroster is on

[1992xlt]

Very strange issue. When the climate control is set to defroster (the blower doesn't have to be on) the engine periodically revs up about 100-200 rpm then goes back to normal (600-700) I noticed there is a clicking sound at the power distribution box below the fuses and it "clicks" right before it revs up. the defrost/heater works fine otherwise. Any ideas?

 

[Tbars4]

..Just from your title my first thought would be a vacuum leak either in the engine bay or the dash and it could be one of the plastic lines..:scratch:

.The clicking you hear is one of the relays..

..I didn't see your location under your name so we can only guess the environment your Explorer is subject to and others in your area may post and be able to better help you if they knew they were local to you..