There are already products on the market that do just that. Solar panel car chargers that plug into the cigarette lighter to charge the battery. Work well enough outdoors from now onwards but useless in mid winter.
The only component needed is a series Schottky diode to prevent the panel leakage overnight outweighing the daytime charging gain.
Mine was from Maplin's going bust sale. It looks after the seldom used runabout car during the summer months quite happily. Though it is as well to turn the engine over every couple of weeks and move it slightly to stop the brakes rusting into the on state permanently.
Snag is some modern cars isolate the lighter when they power down.
Midwinter in the UK even professionally designed solar powered radar signs for "Please go round the dangerous bend" are dead in the water every frosty winters morning. They kill their rechargeable battery every winter too for good measure. There is one not far from where I live.
It lasts about two hours after sunset in mid winter when it might just make a difference to someone coming in too fast to go round the bend. The gaps in the hedging show just how (in)effective it is!