I live close to Boston and this is what I do. Whether or not it's correct...I dunno.
My car is on cement so no problem there but for your car as other's have suggested, plywood/plastic moisture barrier/etc...anything to keep moisture from the ground up.
Up top, I have a breathable car cover first. Due to possible cowl leaks, I then have a sheet of plastic over the car cover. (you know, the stuff that everyone bought when they though there was a thread of a bio-terrorism attack. I still have a lot left..
) This prevents any water/rain/melting snow into the cowl area and trunk, oh yeah, did I mention a possible trunk leak.
Now, loosely on top of the car cover and plastic, is one of those cheap blue tarps. I use cinder blocks on the 4 corners and since it's parked next to a stockade fence, I attach one side of it to the fence to make a "lean to" sort of thing where when it snows, I can walk under one side and push off snow from under the tarp. So 3 loose layers.
As tantransman mentioned, I uncover the car to air it out here and there, of course harder to do when it snows.
I had a mouse hotel when I got the car but I put moth balls (Napthalene) here and there and rodent glue boards with bait and haven't caught one Mickey yet so I think the moth balls are working. Might be an idea to cover/block off tailpipes/air filter snorkel/etc.
Side story: I used to work at Acura and there was an old Acura Legend that had no power. End of story was that a mouse had evidently filled the whole air filter housing/resonator assembly/cold air intake with about 3 pounds of cat food.