Solar is not a bad idea if you have an understanding of the abilities of your application. If your car sits in a parking lot for hours a day, why not use some of all that free energy.
I am pretty sure that most of us here do these mods in part to do our part to help the environment; not only for financial reasons. Personally, I would rather spend my money on anything but OIL!!!
As some one has all ready mentioned, don't under estimate a trickle charge. In my case, pull over 300 watts a day. Usually more than that, but some times less if it is raining/cloudy. That is at 4 to 5 amps. - I use my system to trickle charge my deep cycle that I use in place of my alternator.
Depending on your vehicle, it would not be hard to remove or add your panel. In my case, I used the existing roof rack. It just bolted down to the existing fastener areas. - Probably got real lucky.
Another nice thing about having panels on your roof (elevated ones like mine) is that they help keep the cab cooler by blocking the sun, therefor I don't have to run the A/C.
Now just to be level here, though I have proven the application in a past vehicle, I am still building this for my current vehicle. I hope to be done in a couple of days.
I would eBay for panels and/or check out Harbor Freight. I did both. I found a nice panel for the Previa on eBay and I have a Harbor Freight setup on my barn. Good prices too.
Any who, sorry if I ranted and I did not intend to question any ones opinions. I am simply sharing my personal experiences.
It is a great project to under take. Remember to have fun!!!