Start with a cooling system pressure test with the engine warm and the heater on, I feel that you may have a small coolant leak in the heater core, if there is no leak, there will be moisture entering the cabin around the window or the vents, using your A/c in conjunction with the heater will help remove the moisture on the screen, but this will not fix the main problem.
Mark (mhpautos)
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Monday, December 20th, 2010 AT 7:04 AM