Get a blacklight and shine it into the cowl vent (has to be dark, of course). If you see runs, drips, or errors that fluoresce then you likely have a case of windshield cat wash-itis. It's the devil to eliminate but some well-drooled Nature's Miracle enzymatic neutralizer can work wonders. If that doesn't completely eliminate the smell then your only option is removal and teardown of the A/C box for a thorough scrub and enzyme treatment. If it's cat urine and it's gotten into any fabric, padding, or insulation, you'll likely get a reminder whiff on warm days unless the infused material is replaced.
Brad