You can eliminate that musty, stale air odor that comes from your air conditioning system with a product called AtmosKlear.
AtmosKlear is an odorless odor eliminator and works amazingly well.  You can find it in the Air Freshener Aisle at most Target stores, or order it on line at www.atmosklear.net.  
Simply spray the interior of the vehicle lightly, then spray into each air portal a couple of trigger sprays.  Turn the fan on, pulling outside air thru the system.  Go outside the car to the cowl at the base of the windshield and put about 20-25 trigger sprays into the system.  Let it run for two or three minutes.  Now, go back into the car, switch over to  max air-conditioning.  Put about a dozen trigger sprays in the air in general, and let the vehicle run for about ten minutes.  You are left with a fresh "nothing".
Get some at Target.  It is amazing.

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