Before you do anything you will need to determine the extent of the damage. For sure they have got into the seats. Pull the seat bottom and look at the rust on the coil springs and how much other "stuff" has been been brought in. If there is a lot of rust that will indicate lots of the "liquid" stuff vs. the solid "pellet" kind. The urine is pretty acidic. Secondly you will need to pull the panels on the cowl below the dash and the panels behind the seat thru to the trunk. Latex gloves, full body, arm and leg covering and a good quality mask or respiator are recommeded. If you find signs of infestation in behind the panels I have mentioned...then there is a possibility (good one) that the headliner has also been compromised. My own experience led me to strip the seats and floor coverings out of the vehicle, remove all the panels(including door and parcel shelf), cut out and drop the headliner onto the floor. Vacume all nooks and crannies from the roof down to the rocker sills, then get out the garden hose and start at the top above the windsheild, work along the top of both doors, rear window, and keep working down. When you have done it one time go back and vac. out the stuff you missed and repeat the whole process with the garden hose. Old cars are a lot of fun Eh? Good luck. PS dont forget to clean the top of the glove box liner and heater if equipped. Choose a nice hot summer day with a good breeze. Have an understanding spouse/partner to provide you with encouragement and food and drinks. PSS If your seat bottom is badly contaminated you may need to strip off the upholstry and padding materials and the burlap wrapped around the coil springs. All this is worst case scenario. You may be able to get away with a lot less.