W124 and W201 are very similar. They even share a few model specific parts. W126 is very different from either, though the central locking uses the same principle. In fact, most MBs do share many design principle.
W126 places the pump in the spare tire well. I don't think the pump is exchangable (among W124, W126 and W201), probably not even between early and later W126. In the US market, the 86 and later W126 received many updates. Since W201 and W124 were introduced around that time, I would expect the later W126 has more similarity.
If I remember correctly, I did encounter different wiring for the vacuum pumps though I cannot recall whether they were from different model or same model but different year, and I don't remember which model it was.
Anyway, the difference I notice was the power to the pump. Old design was routing the power thru the door switch. The newer design was that the door switch servers as trigger to turn on/off power which comes directly from battery. Your remote control should work either way.
Xim> I currently own a w124 and a w201. I fitted them with an after-market
Well, I bought a '83 280SEL yesterday, with the climate control (well, it works for now).
I also researched this issue a bit more. Apparently w126's of the first series (before 85) are a bit more complicated. The driver's door doesn't have an actuator, so if you install the remote system. All doors will open except the most important one. :(
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