I would like to prevent it from happening on my pristine cab.

I have cut open a cab or two above the w/s to try to find its start. No luck. I had a theory that it was rain driven from trucks that run highways, as local trucks( municipal) seem to last. I have seen it on trucks without clearance lamps, on trucks from non winter condition parts of the country. I noticed it wasn't too common pre 1986 also. It seems to start halfway in between the gutter rail and top bevel, which on the inside is above where water would puddle. Any one know for sure? Its symptoms are not consistent. Thanks.

 

I think it's condensation between the inner and outer roof.

 

My own observations are that trucks with cab-lights are almost immune from the rust condition, leading me to believe that one should bore some holes (BIG holes, not pin-holes) , just above the windshield, on the inside of the cab.

A draft-tube that scooped air and forced it through couldn't hurt, either.

 

ahh yes never thought about condensate! If a cab were prone to leaks ..ie wet carpet, that would find a high spot to form in warm conditions.

 

I think it is caused by the closed design of these cab-tops.

Fords and Chevys of the same vintage have big open passages a cat could crawl through that lead up into the area above the headliner, thus permitting air-flow; whereas, the Dodge cab-top is two layers pinch-welded together, with an air-space between and very little chance of air changover.