I also cannot find good oem trailer harness for my ML320, 2002, that connects to factory connector. But I hooked up by directly connecting wires. Following is the color on the factory connector:

Red: +12v (1 thin & 1 thick)
Brown: Ground (1 thin & 1 thick)
Gray: Reverse light.
Pink: Right trun light.
Yellow: probably break signal; i don't know how to test this and confirm.
Orange: Break/Stop light.
Green: Left trun light.
Blue: Running/Tail light.

hope this helps.