What
@1967coronet says is right, the heater part of the system should and can work just fine without the A/C element functioning.
Are you talking about de-frosting the inside or outside of the glass?
If it's inside, clean the windshield. You'd be surprised how well that fights mild to moderate condensation on the inside.
Years ago, working a painfully boring security guard over night job, I decided to clean the windows in my Barracuda. This used car has cigarette residue on the windows, particularly around the perimeter of the glass, and you could smell it just a tiny bit when entering the car.
I went through two bottles of windex, that night, and 3 rolls of paper towels. Most of my time was spent cleaning the inside of the glass. Cleaning the rear glass on the inside it a PITA. Just when I thought I had the glass clean, the paper would pick up something and it would smear, again. I kept spraying and repeating, over and over and over, until I could get the glass dry (at night in the cool air) and it not smear and look clean. Don't forget, I was not in a hurry. I was bored out of my mind with little else to do.
In the following weeks, I was shocked to discover how little condensation gathers on the glass when they are clean. In fact, this seemed to last for months. When I did find condensation on the glass, the defroster worked like a charm, within a few minutes on a cold engine, to clear the glass.
Bear in mind, this is in California, where we see very little snow, or temperatures below 40*F. But it was a great lesson. Clean glass doesn't doesn't accumulate condensation (on the inside, anyway) as readily as dirty.
