Kiss of death...this damage is from the inside-out, so what has manifested itself as a hole, is much worse on the backside of the panel. The damage is, in fact, due to the drip rail seal sealer deteriorating, thus allowing moisture to wick through the roof cap & drip rail seam. In order to stop this process, the only sure-fire way to do so, is to drill out the spot-welds around the perimeter of the drip rail, allowing the roof cap to be removed. At this time, you can apply a rust inhibitor/rust encapsulator on the interior surfaces of the roof cap & the inner cab panels.
To exasterbate the moisture wicking problem, there is a mat of insulation in the cavity between the inner roof panel & the roof cap, that when exposed to moisture, wicks it up & holds it, since there is no air movement to dry it out. Also, making the problem worse, mice & other varmets get in this cavity, via the A & B pillars & makes nests, that also catches & holds the moisture.