Not so good for the cats though....sorry, having a hard time with the notion of cats living in a garage. They can be re-trained! First you have to remove all odour of previous pee with Nature's Miracle or similar product. Then you have to attract them back to the litter box using Cat Attract.
Finally, is there a reason why your cats aren't using their litter box? Maybe they don't like the litter, or not cleaned often enough? Sorry, just asking because cats are normally pretty clean unless circumstances prevail.