But why would you even want to remove it? Wax in the car is standard-issue for any surfer. I'm surprised they didn't make it a factory option on the Roxy Edition Toyota Echo.
If you must remove it, there are all kinds of variations on this product, available at any home store, Target, etc. Stuff worked great on our carpet. Not sure how well it'll work after the wax has had time to set into the color of your upholstery, but worth a try.