How big is your solar panel? If it's not quite large, you may find that it doesn't have enough output to run the stock fan. Some people have used solar panels to power small muffin fans that were placed near the ventilation outlet (in the trunk) to try to move some air through the cabin. They don't move much air, but it's better than nothing.
Here's a link to one guy's adventure:
http://www.cleanmpg.com/forums/showt...t=22764&page=4