The first thing you need to do is grind away ALL the bondo in the cab corner and see what is is you are dealing with. That cab corner has rusted that bad because what whoever fixed that corner in the first place didn't coat the back of the panel with a wax oil, or other form of corrosion prevention. When you weld a pull tab, or unispot pin onto a panel to pull the damage out, it burns all the paint off the back side of the panel. And if you do not re-coat the back side after you are done, it will rust..... guaranteed. Alot of shops will skip this step, quite frankly because it is a pain in the ****, and to do it properly you need to pull the interior trim panel to gain access, which includes pulling the back seat, upper seat belts, upper trim panel, rear body trim, ect...... That's why it's a pain. All that extra work to just squirt a little bit of wax oil on it? This is the end result. Most of the time, people do not own a truck long enough for this slow and methodical rust to find it's way through, at least to the original owner. By the time this rust finds its way through, the owner has traded the truck off, and it is somebody else's problem.
If you were the owner of the truck when this repair was done, and still have the paperwork to back it up, the shop who did this repair is on the hook for it. Their lack of care caused this.
Let's assume that you were not the original owner, and you have no idea of the history of this repair. You would have better luck wrapping your lips around a valve stem to blow up a flat tire than to try and start welding up these holes. The rust is far too advanced, and by the time you grind away all the bondo and get into the extent of the damage it will quickly become clear. You need to get a replacement cab corner. First, start grinding and figure out just how much steel you need. Then call a salvage yard near you and find a replacement corner, and weld it is place. To do it properly, you are going to need to remove the box, remove the rear interior, and replace the entire rotted out section. I'm guessing 14" high, and 8" in towards the rocker should about do it. Pretty much cut it at the lines that that picture shows, but a little taller. Do it once, do it right......and wax oil it when you are done.