How cold is it where you are? My '92 used to get frost on the inside of the windshield when it was brutally cold out. There wasn't anything wrong with it and it didn't do it at any other time.
I don't know if this will make a difference, but try turning the climate control off whenever you park the car. It would take a bit to explain, but suffice to say that certain buttons open the fresh air intake vent. fresh = humid also. humid + cold = frost
Whenever my sunroof had a huge leak it would frost up on the inside. I think maybe you didn't close the door, or something. If air and moisture get inside the car then it will frost if cold enough.