After the mess of work I did removing the roof rack from my wagon looking for a leak in the screws holding the roof rack on, I discovered that the problem was not with the roof rack mounting holes at all. I put a garden hose on the roof and crawled inside to watch for leaks. The water was still dripping down the D-Pillar as before so I felt around in the roof structure to see if it was running inside the roof from the further towards the front of the car as I suspected but found nothing. The water literally seemed to be materializing directly above the D-Pillar where there were no screws or mounting points what so ever.
I had noticed in the past that there were a few rust spots forming at the top of the rear cargo area opening. It was beginning to look like my problem was in that area rather than further forward. Talk about a mixed blessing.
I unleashed a grinding wheel on the body and removed anything that looked like it may have rusted until I got to the bottom of it. Ultimately it turned out that the problem was actually in the cars construction rather than any holes that had been put into it. The roof welds on to the side panels and where the roof line meets the cargo area there is a gap. GM seems too cover their weld points with a sealant of some kind. The gap between the roof and the side body panel was about 1/4” in diameter or so and it looks like the sealant leaked. I removed all of the factory sealant and discovered a small cavity leading into the inside of the car and the inside of that cavity was rusting.
I painted the exposed surfaces with Rust Convertor and got as much of it as I could into that small cavity before packing the hole with sealant and giving it all a good coat of rust-preventative primer. Here’s hoping I’ve found it. Tomorrow I’ll show you the surprise that I got while inspecting the rest of the roofline.