Steam cleaning with serious pet de-odorizer. And you need to have it done professionally by a detail shop.
Detail shops have industrial steam cleaners and chemicals. When they steam that seat, the chemcials will soak into the fabric, and into the foam if they pour enough on there (make sure to tell them what happened and that you think it's in the foam). When the steamer vacuums the solution out, most of the cat urine should come out with it, and whatever's left should be killed by the chemicals that are left to dry out.
I used to work at a detail shop, and believe it or not, we once had a customer in who was taking their cat to get neutered. Well, he didn't seem like that idea and sprayed all over the passenger seat. I wish that smell on NOBODY'S vehicle. Anyways, we steamed it, let it dry, didn't work. Steamed it with pet de-odorizer, let it dry. Didn't work. Steamed it with some lemon-citrus smelling industrial strength odor-stopping solvent solution, and sure as hell, the smell was gone. The customer sure was happy when they came to pick up their car.
Trust me, go to a detail shop, the sooner the better.