Go to "The Body Shop" and check out their car diffuser.
http://www.thebodyshop.com/bodyshop/...oryId=cat30070
Works like a hot plate and you pour oils onto a pad and it heats it up. You can buy like 20 different type of oils and it's really effective because you can put it on every few days. I've had so many compliments as to why my car smells so good. It smells a bit girly, but the girls love it.