I have a 1985 GMC Sierra and every time it rains I get water on the floor of the driver side. I have pulled the dash pad off and found water coming under the windshield seal. I resealed the windshield and still have the water issue coming from somewhere. I have read a few forums and found 1 person that had water coming from behind the LH 1/4 fender. Anyone else had this issues and found anything else???

 

wiper cowl letting water in?

 

Because the windshield is not installed with urethane seal and is using a rubber locking installation system it could leak on the passenger side and only enter the cab on the driver's side.
Try sealing the rubber shield seal with a thin bead from a tube of urethane sealer.
Then turn a sprinkler on your truck then get in and watch for a leak