Check the CHMSL. IIRC, there was an issue with those that caused leaks. Cant remember what the fix was though. Maybe the gasket?

Here's a good way to check for leaks - start the car, put the heater/blower motor on high and close all the windows. Get a spray bottle of soapy water and spray around the rear window and third brake light. If you see it bubbling somewhere, there's your leak.