I faced this problem on my 1987 190D. I ended up cabbaging together a diode connection outside the fuse box. It worked fine but wasn't elegant. I never did find the diode and holder. However, I think I know where it is. I'm pretty sure if you remove the knee bolster under the dash on the driver's side and look up to the bottom of the fuse box you'll see the holder plugged into the underside of the fuse box.
I can't check this since my son totalled the car 2 months ago.
1984 300D Euro