It may be too late in the season for you to do this but...
the vehicle MUST be bone dry and clean. Take it to Wal-Mart Automotive and get their rust protection spray. You can choose to just have the underbody sprayed or also body cavities drilled and sprayed so it cannot rust from the inside out. I went for the total job. They use a wax and honey blend that is very effective and does not attack the rubber parts like oil spray does. The product is warrantied. Normally you would do this Septemberish before the wet fall weather really rolls in. If the vehicle has any moisture on it, you are just sealing in the moisture and making things worse. Might be best to just wash lots this winter and do this next year