Try sprinkling baking soda all over the carpet and leaving a box of baking soda open in the car closed up.
I can't remember what the name is but there are products for removing Cat Urine from wood floors and it should work on this too ...seen it on TV so there is something out there, sorry I am not much help. Try calling a Vet and ask about this.. also here is a link that may help.. Good luck.
http://www.catsofaustralia.com/urinestainremoval.htm