I am having a similar problem on my 300TE... shop replaced the pump, and yes it is under the right back bench, but it continues with intemitent problems.

Sometimes it will not operate the central locking from driver's door, but only from front passenger side, another time, not from either door, or rear trunk key.

There are 3 cables sets leading to unit...one has 3 wires, another has 2 and the other has one that plugs into a harness that has the same connector as the 3 cable one. (It has a single white wire pluging into something that has 3 cables)

What and where do I measure?

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Don

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'90 300SL
'90 190E 2.6
'87 300TE (gone to car heaven)
'84 190D (passed along...still in the family and used on a daily basis)