Cab corner question: Prevention not Replacement

[Kenfuzed]

I've been searching the forum for days without any leads. There are plenty of posts about replacing cab corners due to rust but what I really want to know is, how does water get into the corners in the first place?

I'm fortunate that my truck is practically rust free, however after this last rain Sunday I noticed the back corners of the cab behind the seat are damp inside. The truck doors were closed during the rain and was not driven, only sat in my driveway. I don't see anything leaking around the rear window and the door seals appear solid, but they are not new either.

Before this becomes a rust problem I want to identify how water normally makes its way into the corners and what can be done to stop it.