Foggy windshiled

On cool or damp days my windshield fogs badly on the inside. This is an 88T with the auto climate control. The heater and blower both work fine and hot air blows through the windshield vents when set on defrost, but it does not do a good job of removing the condensation; it works, but it takes about 15 minutes to clear the windshield to eye level so I can safely see. I replaced the heater core a couple of years ago, the carpet is dry, there is no odor of coolant in the car, and the coolant level has not dropped, so I am pretty sure it is not leaking coolant inside the car. The A/C unit has a leak somewhere though that I have not been able to find. I think it must be under the dash because I have used the UV test system and found no leaks in the engine bay. After a charge the A/C works ok for a few days but then stops blowing cold. I can live without the A/C but the foggy windshield is a problem. Is it because the A/C has a leak that the defroster cannot efficiently remove the condensation on the windshield?