I find that amazing. Salt likes nothing better than higher temperatures to do it's evil work. A heated garage would be like paradise for corrosion.
Most people don't realize that.
Many also don't realize how much damage water from excessive washing can do in freezing conditions. It gets inside seams in doors, rocker panels, etc, and opens them up as it freezes. This is where most automotive corrosion starts in the north.
It's often surprising to see how long moisture can sit inside of a door or other panel, once it's in there. I have opened up many hundreds of vehicles to repair therm, and am still often surprised to find it moisture inside them days after sitting in a heated shop, or even in the summer, when it hasn't rained in days.
The salt under a vehicle is difficult to remove. But, on a car or unitized SUV, it's not usually going to do more than cause some minor surface corrosion over the long haul. Floors do not rust through from the outside in. It's always from the inside out. There is ALWAYS a water leak which leads up to that. Many water leaks are the result of moisture expanding inside the seams of the cowl during the freezing process, or rupturing the seal on a glass.
On trucks, manufacturers do not go a good job of protecting the frames from corrosion. You can, and should undercoat a frame, if you want to keep it more than 12 years in the north. But, no matter how good a job a rust proofer does, there are always areas inside that frame that are not going to get coated. So, keeping it dry is still your best defense.
When it's extremely cold, if your vehicle is dry you should leave it alone. The salt isn't going to be able to do much when it's that cold. It's also not going to rust the outer painted surfaces of your vehicle just from sitting on there. If your finish is in good condition, the outer coatings are more than enough of a barrier to protect you.
I try not to wash my vehicles unless it's warm, or weather conditions leave them wet already. Then, it's not going to do any more further harm to get it wet.
