The first thing I would suggest that you look at is the recirculate door. I
had a similar problem on our '88 5kCSQ wagon. I can give you more info if
you need it, but the quick check is to look under the dash in the passenger
footwell. You will see the recirculate door through an oval-shaped hole.
If you have the car running with the defroster on but the door is open, allow-
ing air to recirculate, you have the problem. The recirc door should be open
only if the system is set to AUTO w/temp @ LO, or if the OFF switch is de-
pressed. The fix is pretty cheap.
BTW - The climate control does use the AC to dehumidify the air when the de-
froster is running.
I also had a problem where the engine cooling thermostat had broken and was
always open. I was driving on the freeway in sub-freezing air, and the cool-
ant temperature was below the minimum line on the gauge! The coolant wasn't
even warm enough to warm the interior of the car!