I don't know that I'd go so far as to say that there is a problem with your compressor just because you have fogging. For example: if you have a vehicle with no A/C and put it in defrost you are running air up to the windshield to remove interior condensation. To do this effectively without the dehumidification of an A/C compressor you have to have a good volume of air moving against the fogging surface, and it should be sufficiently heated; don't ask me the temperature delta necessary to fend of condensation, but that is generally what has to occur. If you have a vehicle with no A/C and have fogged windows and the vehicle is cold, it may be a few minutes until there is enough engine heat being generated to heat the air moving over the surface of the glass to effectively remove fog.
Blowing cold, but conditioned air against a fogged glass surface will not immediately remove the fog. You have to have some sort of temperature differential in order to remove the condensation in a quick manner. The compressor helps by dehumidifying the entire cabin of the car, thus reducing the humidity in the cabin that can become condensation on the glass, thus reducing the need for a heavy volume of air flow and greater than normal heat levels. However, there has to be a change in the surface temperature of the glass to remove the condensation. This is demonstrated two ways, via heated air on the windshield and front windows of the vehicle and by the heat strips on the back window. The heat strips work regardless of air volume, or air temperature because the glass is being heated directly, but where air movement is employed, the volume of air has to be heated to accomplish the same feat.
If you do have a problem with your A/C compressor, it will show up in the summer when you want to cool the cabin and you get no cold air out of the dash registers. Until then, if you have a lot of interior condensation, increase your cabin temp, increase your fan speed direct all your air volume to the defrost output and wait with engine running, only if necessary, for the engine to start producing enough "waste" heat to provide sufficient temperature differential to do the job. Good luck!
