After a solid six weeks of hacking at it in my spare time, I've finally gotten the driver's-side rear door on the Mercedes open.
The culprit? A rusty mechanism and a jammed child lock. In the end, I had to pry the locking tangs away from the assembly which unhooks the catch. And since the door had been locked closed for so long, it took another few minutes of pulling and kicking on the door until the weather seals finally let go.
Once I had it open, it was a fairly straightforward matter of unbolting the assembly from the door (thanks, PB Blaster) and then working some penetrating lube through it. I shot it down with some WD-40 to kick any recent water out of there1, and put it all back together—and now it works like it did from the factory.
1 Say it with me, now: WD-40 is a lubricant, but it shouldn't be used as a lubricant. WD-40 is a solvent, but it shouldn't be used as a solvent. The WD stands for water displacement, and WD-40 should be used as a penetrating water displacer2.
2 Duct tape should really only be used to repair ducts. I mean it.
I left a comment on Mike's blog about the cockPit that you would do well to read.
Mike's blog?
Also—drop me a line at dan.driscoll@hotmail.com if you get a chance. It's been too damned long since we last talked, and I'd like to catch up with you.