urine neutralizing sprays are available - you may need to disassemble the car to get to all the stinky parts. Other option - sell the car after coating the ac vents with aerosol spray.
You can remove cat urine from your car seat by applying a product called D-Molish. D-Molish actually neutralizes the urine smell, it will be totally gone!
Cats pee on cars, and other objects, to mark their territory. An unnuetered male cat will do this more often, as their natural instincts tell them to. Neutered cats will do this, but much less.