Yet another thread asking about a leaking cab! I am on the wet coast
(Van Isle) we get alot of rain (understatement)
I had a leak that was caused by a poorly intalled Lunde visor that
after lurking on the forums thought was
caused by the cowl crack or the windshield , anyway that solved thought I was done with it.
Not to be, I then had a leak coming in the back of the cab (2 years later)
Discovered the plastic tray behind the seat full of water...
I siliconed the split rear window shut . I caulked all around it where the glass meets the rubber gasket.
Still leaking the rear cargoe light was also caulked.
Still leaking (but not as bad) my only area that is left to try is where the rear window rubber meets the truck body.
Did I mention that I hate leaks?

 

What year/model truck??
Check the cargo light seal and lens,
Check the seals under your clearance lights( if ya got'em) as well as check for cuts or cracks in that seal around the rear window
They aren't all that expensive to replace.

 

The truck is a 92 w250, I will caulk the lens on the cargo light.
I don't have clearance lights, the
seal seems ok for cracks, what about the drip gutter?

 

Rusting out along the drip rail by the windshield is common, but I haven't heard anyone say that made it leak in the rear of the cab

I'd go light with the caulk, you may want to change that bulb someday...