defroster problem
 

I have a 1995 Jeep Cherokee and need to know where to start looking for my problem. The defroster will blow on the floor, and will shift to the dash vent, but will not move to the top dash defroster position. It moves from floor heat to dash vent without a problem but won't shift to defrost. Has anyone else ran into this? can someone point me in the right direction on where to look? thanx