Easy ways to check Main harness is to cut away black sheething at connector at thermostat water temp sender (pik) or remove spark plug cover. Another is the "B" harness (above oil filter) for oil pressure sender & starter (pik).
For new part number, ask MB dealer. Part number on cable at connector. Remove battery and plastic cover behind it.
Keep searching "m104 Engine Wiring Harness"
ex
http://www.peachparts.com/shopforum/showthread.php?t=149851
Bob