I’ve experimented with assorted solar energy collectors (electric and water) for various applications over several decades including charging the car battery. Most recently I tried a Noco BLSolar2 on my Mazda3, it worked via OBD2 but had nowhere near enough output to replace almost 1 Ampere-hour a day of dark current and battery self-discharge.
My conclusion is that to extract worthwhile electrical power here in UK you need to have something massive, several square metres, which is far bigger than the majority of toys being sold to go inside a car.
The time our little cars need them most is during winter when the car is least used. However this is also when solar is least effective, with weak low-down sun and short days. And then there is the conversion efficiency of the panel itself depending on type, varying from abysmal to poor.
The figures quoted for the majority of commercial solar chargers are for basking in bright midday sun right overhead cooking down from a clear tropical sky. In UK at 50degrees North in winter we might be lucky to catch even a tenth of that irradiance peaking briefly at noon in a short day, even if the panel is perfectly aimed with no glass in the way.
If you count the Ampere-Hours in a day (how much recharge the battery might need), and compare a normal grey UK winter with that blue-sky tropical location, the total amount of solar insolation (ie joules to produce a smidgeon of charge) from the same panel perched inside the car on the dash-board in the UK is likely to be less than a hundredth of what is possible lying on the beach in the tropics!
One possible idea is to have a secondary battery or power bank that can be easily moved from house to car and connected to a suitable 12V to 12V Smart charger for a week or so at a time as a maintainer. But I’ve yet to find one that is properly Smart and more than just a simple jump starter, not that I’ve been looking all that hard.
A better idea is simply to have something like perhaps this jump starter (but I’ve not tried this one). Then use it occasionally to make sure the car can start without caning a nearly dead battery, and take the car for a proper run to make sure everything is exercised, all moving parts and not just the battery.