My late model S10 has a problem with that too – even on nice days with only slightly cooler air temps. It was particularly annoying because I have a 4 cylinder and every time I turned the defroster on, the engine’s power was greatly reduced (it is designed to automatically turn on the A/C to dry the air with the compressor). I found that Rain-X Anti-Fog works well enough to almost completely eliminate fog. It’s instructions call for a reapplication every 30 days, but it’s so easy that’s no problem. It’s not so expensive and usually not hard to find in automotive and department stores (in the US anyway). BTW, if you haven’t tried the original Rain-X glass treatment (goes on the outside of the windshield) you’re missing out. It causes the rain to bead up on your windshield, and as you drive, the wind blows the drops right off. That’s a way cool visual effect and works great (at a moderate speed … >45 Km/h ~ 30 mph). Link for Rain-X Anti-Fog