If possible, mount junk wheels and tires (spare tires?). Wrap a cover under the car when on the open trailer (keep most of the weather from blowing under the car.). You can do this before you put the car-cover over the top.

Antifreeze also will reduce the rusting of the interior of the cooling passages. In the spring, you can drain it out, save it and use it the next winter.

Moth balls in the car's interior.

Try a can (pick the appropiate size) over the open end of the exhaust pipe.

Put the battery in a warm place, and use a new generation trickle charger (Battery Tender, etc).

Bill