Where is this interior leak coming from?

[C10SLEY]

When it rains, passenger side gets wet down in the corner angle where your right foot would be. Looks like it’s coming from around the heater core on the right side. Any ideas where it’s coming in from on the outside without me tearing everything apart first?

Thanks!

 

[ali_c20]

Test it. Pour water over the Windshield, if it stays dry pour water in the cowl. This way you can find where the leak sits.

 

[Grit dog]

Assuming it’s water and not coolant, my money is on from the seam in the cowl and possibly blocked drain hole in the bottom corner of the cowl.
As said above. When it’s dry, start selectively pouring water to help pin point the leak. Take the valance off the cowl so you can see in there. There’s a transverse seam that I believe tends to be a common leak spot. Or rust through hidden somewhere in the cowl or bottom of windshield frame (under the gasket).
Your truck looks real clean though, so not sure if it has any of the damage I mentioned above. But you didn’t mention how it’s been stored over the years.

 

[gmbellew]

The cowl drains get clogged eventually and water stacks up and comes in the kick panel area. Under the truck behind the front fenders is where they drain. You may have to remove a plastic trim piece and pull some foam out of the way. I've had to clean, flush, clean, flush, etc until I got all the debris out.