Fogging from the inside is 99% of the time just moisture.
Take them apart, don't stick them in the oven, you'll melt the housings. Just use a hairdryer on full heat and work your way around, peeling the lens from the housing as you go. Then clean the inside of the lens. And make sure it's bone dry before putting them back together. Use an all-weather external sealant to seal them back up. Leave them 24 hours to cure, then reinstall.
If the outsides are fogged up still, then thats easy to fix with the sandpaper and meguiars method.
Here's a thread showing you what to do for the outside...
http://www.civiclife.net/board/topic/57 ... eadlights/