I would take it back to the dealer and tell them the stupid thing is rusting and to take it back. Maybe that way they will do something for you free of charge!! Afterall, you did spend alot of money to buy it!!
However, to stop any car from rusting in the future it needs to be washed where the under carriage is any time you travel over salty areas be it salt on the roads for snow, etc.
Was it a used vehicle?