Inside, remove both AC filters, and then there should be 8 screws holding the inside plastic shroud on.  Once those are all out it will come off and you'll see the 4 bolts.  Notice the rubber foam gasket there and look for any tears or rips in it, tightening those 4 bolts will almost assuredly stop your leak when it rains.

A question about the water:  If you had the AC turned on during a rain storm, would there be a possibility of condensate dripping out of the unit onto the floor?  I've had some car AC's "spit" at me in very humid weather.  Just curious and here in NV we don't know what humidity is.