Rear Quarter rust and water

I had a little rust coming through on the left rear quarter panel so I had it repaired a couple months ago. I now see a small bubble starting to come through already in the same spot, so I did a little more digging.... I pulled the interior paneling out to find standing water at the bottom. There is also all kinds of rust. Seems the water is sitting and eating away at the metal from the inside out. I dried it out and have checked on it when it rains and it appears to hold water anytime we get a fair rainfall. I have tried to observe where the leak is coming from with the back end closed up and wife spraying it with the hose. I can see a small stream coming in from behind the tail light, but cannot figure out for the life of me where it's getting in. Is this common since so many of these cars rust out the rear quarter panels? Has anyone pinned where the leak comes from? I'm contemplating drilling a couple holes for it to simply drain kind of like there is under the spare tire....

My Caprice was leaking in that same area. Was coming through cracked Seam Sealer at the rear of the quarter panel, behind the tail light.

There is a drain in that area that the factory was kind enough to block with seam sealer. Clean out the drain with a small screwdriver and the ponding water issues will be fixed.

Thank you for that. I'll pull the light and check out behind it.