When I lived on the tundra of New Jersey, I washed my car during the winter as much as possible--whenever I could face being cold and wet (sometimes I could see a water film freezing before it had time to drain off). That included aggressive spray-nozzle work underneath, up into the rocker panel drains, in the cowl vent, into the radiator and headlights, into the little drains at the bottom rear of the doors, etc.

I was there only two winters, but maybe it helped--I've still got that car 15 years later, with minimal rust.

Good luck.

Doug Harvey