I used to sleep in my truck often enough when driving for a firm in Poland
It's also recommended for truckers to pull over and a take snooze, when feeling a sense of fatique.
do not do it in a paid private car park
Are there many inexpensive outdoor camping areas in Poland? Folks may considering paying a small fee, in order to camp in/out of their vehicle for the night.
you are sleeping in your car in winter , you need an army sleeping bag thats good for minus 30...
Probably a good idea to bring along a thin solar blanket too, for extra warmth.