How do I fix an air vent that leaks extreme water on the passenger side of a 99 crown Victoria p71? Please help!!

I would pop the hood and look around the top of the engine bay for debris, leaves, blockages, etc. Then look to see if there are any drain holes (there may not be any) that might be clogged and overflowing to places water isn't supposed to go. I don't know anything on the design there but it's worth a look.

I had a town car with this issue. I think they are the same there is a cabin air filter near the cowl. I gasket around the lid deteriorates then leaks. Take the cover off and put a bead of rtv around the lid. Reassemble.