I just experienced a similar problem on my 85 4Kcsq, only on the right side.
It was leaking between the windshield weatherstrip and the body. Very
difficult to see. Found my leak when my car stopped running when I tried to
accelerate hard one rainy day. Limped to dealer, popping & sputtering. Next
day problem discovered... water in main computer...just add goldfish...I mean
full of water. $$$$ later for new computer and trip to glass shop all is
fixed. Now the passenger floor is dry again, too.
Best way to check for that type of leak is to remove driver's windshield
pillar cover. Carefully pull door weatherstrip away from pillar area and
remove cover. Add copious amounts of water to windshield area and observe
body to weatherstrip area for leaks.
Two other areas are known for leaking. The fuse box and the body seam at
the corner of the windshield.