I'm sure it could be done (in Canada loads of the cars had flexes with
what looked like mains connectors hanging through the front grilles for
connection to engine block heaters) but the question is why? Surely it
would be easier to have a fused 12V connector at the car, so you are
less worried about insulation/weatherproofing/safety concerns etc. This
is the system loads of motorcyclists use with smart battery chargers.

Its done in this country with fire pumps, so that they are ready to go at a
moments notice. Mains is better for current load.

Dave