Right in front of the rear wheel, near the door, the seam where the rear quarter meets the rocker panel.

I saw there's a weld/braze on each end of the seam and I removed them but it still hasn't came free. Am I missing a step or do I need to just play with it a little more?

 

I figured it out pretty quick after getting a nights sleep. There's a few spot welds inside the panel attaching the quarter to the rocker. You can't get a drill in there, I used an air hammer between the panels to bust the welds.

In case someone needs help and stumbles upon this one day.