So according to the diagram I posted above, constant power (F1 term 30) to the relay coil passes first through fuse 12, then through the left door switch. Switched power (F1 term 15R) goes straight to the coil. I'm stepping out to verify all this and will report back.

 

 

A few pages later, in the same document package, I find this interesting revised drawing of the same circuits.
1 - As denoted by the revision note 2 and 3, it's hard to imagine that after 1990 the only way to power the K5 seat relay was to open one of the front doors, but this is what's indicated. If you remove the circle-2 wires and the diode, then the only way terminal 4 of K5 gets power is through the door contacts. I'd love to hear anyone else's interpretation of these revision notes.
2 - Can anyone clue me in on revision note 1, that the K5 relay switched from a code-D to a code-A in 1997 and beyond? What is the difference a code-a and code-d relay?

 

What you're not getting is the seat diode. The door switch will provide power when the door is open, so that you can open or close the window as you leave the car. When the door is closed, only switched power reaches the window circuit. The diode sits between the two power sources and prevents power from backfeeding through the entire electrical system when the door is open and the switch is off. The blocking diodes are usually located on the underside of the fuse box, accessible by crawling under the steering wheel. More here:

Seat Diode

My power seats work when the doors are open. But when the doors are closed and the switch on, they don't. I'm quite sure the problem is the diode that I've circled in the attachment. But where is it located? I can't find it in the locator charts in the FSM, not sure where to look

www.benzworld.org

 

Thanks for the reply. I certainly understand how a diode works, and how the dual power feeds work. Maybe you didn't see revision note #2 in the second diagram and my comments in the 5th post. It seems to indicate that the diode and those two indicated wires are only present before 1989 which I don't think is the case.

I now believe that I do not have a problem with the seats or any associated wiring, only a slowly failing battery that is past due to be replaced.