if not coming from the coolant (you could check your level at the reservoir), smell it to confirm that it is rainwater will suggest some seals...we could chose already molded to fit, or spool, and you cut to fit~ will begin shopping.....now~
here's some jazz about weatherstripping...cut to fit~ http://www.jcwhitney.com/fairchild-industries-glass-weatherstrip- seal/p3039069.jcwx?filterid=c3813d2168u0j1
Could also be the AC drain, if it doesn't smell like coolant.
I would go with the A/C drain tube. Locate it under your car. Then take a stiff wire ( like a clothes hangar) and run it up into the tube. Most likely it is plugged with mud or an insect.
have your mechanic "pin blast" air through the drain~ this will clear out the spiders and debris~
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