[ItWillBeSlow]

Ive been working on this repair and thought I would post an update.

I cut out the interior B-pillar lip to see what caused this. I had a ~1" and ~.5" patch of rust between the panels where the rust came through the exterior and where I blew a pinhole. Other than this, the metal above and below this area was clean. There is some sort of factory sealer/adhesive here, which you can see in the photos. It looks like a ribbon on the interior of the panel, on the cab metal. I'm not sure if water somehow got behind this material over the years and caused this issue, or what. I cut out these areas and decided not to dig any further since it looked clean and I did not modify or patch any other metal in this area.

Some pictures of interior and exterior where I cut out - 1st pic shows the rust from the inside' 3rd and 4th pic are patched areas. Luckily I had this metal leftover from the cab corner.