purchase a spray called 'urine off' which you can get at your local vet or maybe even a pet store. it should get rid of the smell/stain. it retails at my work for $22.30, have had some very good feedback from it. no gaurantees though
next time transport the cat in a cage. u can also purchase a grill which sits on the bottom of the cage. put a towel under the grill. that way, if the cat does piss itself, the towel should trap the piss, and you'll minimise the chance of the cat sitting in it's own urine. or you could go even further and cover the car seat in towels for extra contraception