Cars are a great place for solar. The military's been using them for years. This was my driver tracking mud all over the "No Step" sign, but the little solar panel shows up pretty well. They get parked for long periods, sometimes with the radios on all the time. No problems, at least with the batteries.
Don't scoff at something being "only" a trickle charger. I slapped one of
these on my shed once the snow melted and by the time I needed to use the mower it worked. Usually I have to drive out to the shed or push the mower back to the house so I can jump it the first couple times I use it. This year it started right up- not too shabby for $25.
Depending on your usage and where you park during the day, a DIY panel could replace your alternator:
flexible solar panel - Google Search. I don't think it would help me too much, but you could probably turn your whole roof into a panel without having to know too much about electricity.