If you're planning to use the solor panel to trickle charge the battery for extended storage, but there's no appreciable load on the battery, a very low power solar panel will do. ICP Solar http://www.icpsolar.com has a 2 watt unit for the dashboard for $35. You can buy the same one at Boater's World, but it's marked up. That one is not rated for rain exposure though, so don't use it on your tractor or anyhting like that.

If you don't want to rewire the inside of your car to get hot leads, remove the bulb from your dome light, clip the leads there, then turn on the light. This method also provides a fuse between the panel and your battery, which you will need anyway. Be sure to use a multimeter on the light socket first so you know which contact is positive.