Cat Piss.

Best remedy if the spray is localized and findable is to mix 1 pint of hydrogen peroxide with one tablespoon of baking soda and one teaspoon of dish detergent or laundry detergent. This solution is an organic reagent. Search for the place/s the cat sprayed, shiny splotches with drippy marks. Wash area with the solution rinsing the rag often with fresh water. After washing saturate the area with the peroxide solution and let dry naturally. For carpets saturate the carpet deeply, let set a half hour and wet-vac the area thoroughly, repeat a few times to dilute the urine then let it dry naturally letting the peroxide do its work. The peroxide will oxidize the organic materials in the cat spray. After cleaning with the solution straight hydrogen peroxide can be poured into spots on carpets and left to soak for awhile before adding water and sucking the whole sordid mess out with a wet-vac. Upholstery can be removed from the seat frames and cleaned successfully with the above solution. So can the foam but that will take a bunch of rinses if you are up to it. If the cat spray has been in the car a considerable time the odor has permeated the hardboard door panels, carpet, glove box, padded dash, headliner, and shoddy insulation, after cleaning only time...a great deal of time, years usually... or removal and replacement of the fibrous materials will get rid of the odor. In other words, if you need the car next month you'll have to gut it out and replace everything cloth/vinyl/insulation/carpet/headliner/shoddy. Sorry, that's just how it is.
Best future defense? Keep your windows rolled up to just a 1/4 inch crack, doors and trunk closed from now on to keep the buggers out. And yes, I too support the disposition of un-collared felines found roaming about, there seems to be no end to them, my garden is full of small bones. You get a couple or three randy toms roaming about your place and all hell breaks loose with the marking, what with every female in the neighborhood joining in the "fun".
Yes I have a house cat, and I like Pepper very much indeed. She never goes outside, and when she dies I will adopt another starving orphaned kitten. The strays though have to go.