Hmm... maybe part of the reason you haven't had that expense with your 1987 Corolla is that (if I'm not mistaken) your Corolla has
manual door locks?
So, the good news is your MB is working just as well as your Corolla that you like to compare it to... just reach over and pull that lock up by hand!
I don't know why MB uses vacuum door locks. I had a VW Jetta that also did if I remember correctly. Maybe in theory it is more reliable than an electromechanical system, though I don't know if that's proven true in practice. Or maybe they just do it to be different... nice smooth movement instead of the nasty inelegant electromechanical clunk.
I'd be curious to know the reason too.