I woulld guess, it would have to be another cigarette lighter which is "live" all the time.

A fused wire fed thru a grommit,
then you could leave the solar charger in the car.
they are quite easily bought from fleabay etc

or.. you could, if it's somewhere out of the sun,
have a spare battery, inside the car
and connect that to slowly charge the original in situ.

when I was running an 12v to 240v invertor,
to power a laptop and 2 printers in the boot of my car.
(which were printing flyers , while I was at a customers,
I split the 12v cigarette output to power 2 batteries.
ie the original battery under the hood, and another battery in the footwell .
(or the boot etc)
I know there is a slight risk of hydrogen gas... maybe someone who knows better can comment
but I never had any problems, doing this daily for months on end.

Are you away from the car?
as you can also get an alarm, that tells you if the car battery is low
which would prompt you to swap it over, or recharge it.

or , every couple of months bring a fully charged one along and swap it over... the duration depending on the parasitic drain.

Cheers
Liam