Originally Posted by Bio300TDTdriver

I'd be willing to bet that the rust goes under the rubber. That is where it starts in the seams between the steel. There is a good write up on the repair of said rust in that exact location. My 124 had the same bubble and when I pulled the window out I found more on the seam. I would fix it sooner rather than latter. AFAIK rust doesn't sleep.

The rest of the car looks to be in fantastic shape.