Urine = Acid. What better to neutralize acid than good old baking soda. I'd first take the car to the car wash, empty the trunk of it's contents, then stick the wash wand as far down as you can to blast out any junk down in there (you'll likely be surprised at what surfaces!). then dump a box of baking soda down each side liberally add a bit more water to activate it, and let it soak a while (30 minutes or so). Then wash it all back out. Now is the time to spray the metal down with the product of choice, The Krown stuff works good, tranny fluid or motor oil as well but they wouldn't be my first choice. I'm with the "POR15 is garbage" crowd.

Also. when rinsing the inner trunk/wheel well areas, watch that you dont blow water forward and soak your carpeting in the rear floors, you dont want a car with wet carpeting = Rusty floors will soon follow.