I spent a lot of time looking for this diode on my 190D. I tried and tried to unbolt the electrical center (fuse box) and reach underneath. I couldn't get it done. It wouldn't lift high enough and I was reluctant to start pulling the wiring harness plugs. In the end I jury rigged a diode to simulate the effect of my blown diode. I later learned that you can reach this diode by reaching up behind the dash after removing the panels below the dash and above the foot well.
I'm a chemical engineer not a EE but I think the normal failure mode for a diode is to fail open not short. If that's true this diode may not me the problem. Good luck!
1984 300D Euro