Ive seen flexible 100 watt panels on ebay, however they are in rolls that are 16 ft long, a foot across. Id have to run that length wise across my car bumper to bumper.
I saw go for it, panel to battery using blocking diodes if they arent already built in. Im using a power supply to supplement my charging system, 150 watts and got 6 mpg in my honda. I recharge the batteries at home.