If this only happens during heavy rains, Take it to an auto glass place, ask them to take out the windshield and reseal it.. that might cost alot for them to do it, not sure..my parents had the same kinda prob on their 2000 grand caravan, it turned out to be that the venting on the outside of the van (where the wipers go on)was clogged with debris, theres a drian for that it may be clogged and backing up into the cab... Hope this helps ya sumhow..

 

I have the same problem, I have a 2000 blazer with a wet spot on the passenger side foot area. Ever since I had my car audio stuff installed its been wet. I checked underneath my car, no holes. I only use my blazer on weekends too and it remains in the garage on weekdays, could it be moisture building up on floor or too many floor mats trapping the moisture? i dont know.