Thanks for the explanation Steve.. I have come to the same conclusion. My problem is that I cannot trace the wires. They disappear under the floor of the front seats. There is a junction somewhere named X48F. It's a distribution center or hub for the red/white wires which carry constant battery voltage for the components of the central locking system.
I have taken the car apart looking for it. Panels off the doors, pillars... removed back seat, taken up the mats and sub flooring and I cannot find X48F.
I feel rather certain that the power loss is related to that junction.
I have located the junction for optional equipment under the driver's side kick panel. The power breaks down in the wires between that box and the passenger door (no voltage in red white). I have current in the driver's door and the trunk... also at the pump. The passenger door fails the unlock test when testing control voltage.
Will check it further tomorrow. Can you look up the location for X48F and verify its location in an 87 300E?
1993 190E 2.3
2000 Toyota 4x4 Tundra