Thanks for this writeup, helped me out a lot.

I did find a perfect harness for this type of install. My car is the same as yours' E300D with the becker 1492. Found a harness at Fry's Electronics.
On package is Metra, Turbowire. 70-1786. Says is for Mercedes 94-04 and Land Rover 99-04.
The wire locations and their usage exactly match the factory Mercedes harness.