First, what is the parasitic drain of the car?
By that I mean the Engine ECU, Alarm, Radio, etc. It should be 30mA,
but some cars can be a bit higher.

Second, will the panel be enough in overcast winter?
30mA sounds little but over 24hrs totals 8.64 watt at 12V. In overcast
winter your 10W panel may produce only 2W for a few hours a day. The
car windscreen reduces light, the angle of the panel and coverage
provided by the car reduces light further. So you might get just 4-5W
on the worst days which means a shortfall.

A 10W panel is not cheap, a 15W panel is getting quite costly.
If you have a high parasitic drain such that the car battery goes flat
in 2wks, they can be useful when parking at airports or for tipping
the balance back towards tolerable.

Would be better to connect the battery to something like a CTEK smart
battery charger / maintainer. They are about £45 for the multi-LED
version. They can be left connected outside (waterproof), but require
an equally waterproof mains connection - so best put in a shed or
garage with the battery.