If the concensus is that a cat really did urinate in you air intake, my first thought would be to flush water down the air intake to get rid of any remaining, uh, "fluid" in the plenum chamber. Maybe pour a gallon bucket of hot soapy water in there. This in itself may fix the odor. As a matter of fact, if this is what happened, why would it occur only on A/C? It should be coming in through the plenum, not only on the evaporator.
If it's only on A/C, then a stinky evaporator would make more sense. There are sprays you can buy at Auto Zone, Pep Boys or someplace like that which treat this problem. The usual course is to turn the A/C on with high blower speed and spray this deodorant/bacteria killer into the plenum, then shut the car off and let the spray do it's thing for a half hour or so. You could also use Lysol if you don't mind the smell. I'd do the water flush first though, just in case.