On my 1987 190D the electrical manual clearly indicates that you should be able to adjust the seat with the door closed and the key in accy, run or
start. The problem was finding the diode on device xF. I never did find
it but I've deduced that the devices that start with a small x are on
the bottom of the Electrical Center (EC). I went looking for it but I couldn't lift the EC high enough with the harnesses plugged in to see or reach it. I was reluctant to pull on the wiring connectors - they seemed pretty tight and probably have some sort of latch. I've since learned that you can reach up to the back of the fuse box from under the dash. I'll do this next time if I have any problems with my current setup.
Not being one to give up - here is the solution I came up with. I purchased a 50V, 1A Micromini Silicon Diode from Radio Shack. Cat No. 276-1101A. I trimmed the leads and crimped the anode to a ring connector lug, crimped the cathode to a butt splice, heat shrank this section, ran a 14 ga (a lot bigger than needed) from the other side of the butt splice to a Tap-In Squeeze Connector. The other half of the squeeze connector was wrapped around the wire that supplies power to the power seat relay from the door switches. I bolted the ring connector to
the unused 15R terminal on the lower front of the electrical center. On a W201 the power seat relay is K5 but seems to be in different locations on the Auxiliary Relay Box with each model year. For an 1987 W201 the applicable pages in the Electrical Troubleshooting Manual are 103/2 and 138/10.
On the W201, bolting the ring lug to the terminal is easier said than done. To my great suprise there was no nut behind the terminal. I had to file down the flats on a nut to get it to slide behind the terminal. You also have to unbolt and unscrew the electrical center and lift the front to reach the terminal. There has got to be an easier spot to tap into 15R. Even 15 would work. As far as I can tell the only difference is that 15R has power when the key is in 3 positions - 15 only in run or start.
At any rate, the job is done and seems to work fine. I'll let everyone know if any problems crop up.
1984 300D Euro