Cigarette lighter?
Many of those sleep when the vehicle key is removed.
Also you will need the solar panel to be outside of the car. You lose 40 to 60 percent of the power through glass. And as much again if it isnt facing the sun.
And a 50 watt panel is too big for inside.
To find a good one I used ebay, bought half a dozen and watched/measured what they did.
You dont want a 3 stage charger one. You want one that just goes to 13.4 or 13.8 in winter, as you choose, and sits there when the sun hits the panels...
I will see if I can see a link to a similar one.
Mine looks exactly like this. It does what I wanted. I threw at least half a dozen away that didnt...
https://www.ebay.co.uk/itm/MPPT-10A-20A ... Swy0JZ5az2But it depends on firmware and any programming changes thay make so no garantees.
If you dont want to mess about you need a 50 watt solar panel on roof. Stuck with clear silicone rubber. And set the correct voltage. And connect permanantly in the rear of the van.