I have pondered (and posted) about a few ideas regarding charging the battery of your car with solar power rather than with the alternator, except when necessary. Here is a solar car-battery charger that you can get today. It's called the Sunsei SolarCharger, and you mount it on your windshield where it can get good access to the sun. I suppose that it would work on the dash or on an open sunroof as well. The charger features a trickle charger when the battery reaches its full charge, so there is no worry about leaving it hooked up for too long. I use a trickle charger on my motorcycle when the weather turns cold; this would work out just fine for me, if I were able to mount it outside the garage and run the cable inside to my bike. I think that I may just have to look into one of these for myself! Anyone else out there?
If you want one, I did a quick search and found them here. At that site, they range from $40-$160.