My car is kept inside undr a cover (Tyvek) and after a while it develops a "stale" smell. I ran into this product while on vacation last month. I put it into the car and it works quite well. It's also inexpensive.
Things that can produce odors are moisture, rodents and any organic material left behind. Are you heater vents closed?
Odor Eliminator:
http://www.gonzocorp.com/odor.htm
"The volcanic crystals have a negative charge that traps positively charged odor particles. The process is similar to a piece of metal being attracted to a magnet. These products are ideal for ridding your house, refrigerator or car of unpleasant odors. The Gonzo Odor Eliminators are also long lasting. The crystals can be recharged by placing them in direct sun light for 6 hours. Once recharged they are as good as new."