wash the undercarrage during the warm winter days
As long as the temperature is below freezing, salt won't corrode your vehicle. It is when the temperature starts to get warmer and the salt is mixed with water that it becomes a problem. As soon as it starts to thaw, get to a car wash (Go to a hand wash place; not an automatic) and power wash every nook and cranny under your wheel wells and bumpers. Be sure to get those little pockets around mud flaps and fender flares where dirt and salt can accumulate.
Some states will also mix salt with coal cinders to improve traction. Coal cinders are nasty too, because they can get kicked up by tires and cause minute chips in your paint; especially along your rocker panels and lower doors. The best defense against that is to stay in the garage.lol
A good coat of wax in the fall before the bad weather starts is a good idea. If you insist on aftermarket rustproofing, this is a pretty good product
www.ziebart.com/