window fogs

we bought my son a 99 sc1 for a first car. Not a perfect car but we intend on restoring. It gets him from point a to point b since the weather has gotten colder he notices it stays fogged up on the inside longer. almost seems as if defrost just doesno't fix it. In the summer the air workds great. any ideas

Is the slider working? Can he feel heat coming from the defrost vents? What is the operating temperature of the car? More info is needed.

Make sure he doesn't have the recirc on. That will fog up the windows.

I have the same problem on 1995 SL1- seems I have a pinhole leak in the heater core! I have cleaned my windows 100s of times- vinegar-alcohol -etc- only the change out is going to fix it for me!