Does washing the car in the winter even help?
If you are running through an uncontrolled automatic wash you're dissolving the salt in the water and possibly forcing it into places that normally wouldn't even be exposed.
Do you think these places are generous with their water and all of it gets washed away?
Afterwards your car cannot dry easily since its the winter. Corrosion forms in the presence of moisture, salt accelerates it.
Even if you manage to rinse the salt off, how long will your car stay salt free?
I don't know, periodically washing the car in the winter to prevent corrosion just doesn't make sense to me.
It makes sense to oil spray it to lock the moisture away and leave it until spring.