I've got a '99 Chevy 3500 that has a water leak on the passenger side. It is coming from the far right side under the dash, but I can't peg it. After pouring water around the area, it seems to be coming in when the water goes between the front right fender and the door jam. From the inside it is coming in from high up on the upper right side of the floor (behind what I would guess is the heater blower). It is coming from really high up, but I don't know where to look.
I really don't want to have to take the dash apart to trace this. But I've been under the truck, down under the hood, under the dash and I'm stumped.

Any suggestions?

Thanks in advance

 

Check the area where the heater core goes through the firewall. There should be some insulation around the pipes to keep the water out.

 

Did you fix your leak? I have a 1998 GMC Z71 and you described my leak exactly. What was your fix? Thanks, Chet