To bring closure to my post [for those who use the search feature]

I ended up taking the car to the MB dealer.
The problem ended up being under the fuse box, in the terminals leading to fuse #7 (for those of us with a W126). There was a small crack which they soldered back, and Whala! everything works like new. That explains why both windows and seats were affected universally.

Lesson learned - don't be forceful when working around these older electrical parts. Not certain that I caused the break, but I will exercise more caution in the future now that I know.

$75 later, and I was on the road again.