There are two actuators in the rear a/c one controls the vents floor/ceiling the other controls the temp blend. The easier of the two is the vent control unit located directly under the cup holder in the rear, this can be replaced by removing the cup holder and junk tray. The other unit is behind the rear quarter trim panel. Just pop the rear cup holder out and feel the white box located under it and see if it feels like the ticking is coming from that unit.
Also 96mustangs advice about the 1st gen gears can save you some money, I recently picked up six sets of gears at a u-pick salvage and was only charged $5.