I bought an OEM Mercedes main engine wire harness from Phil for my 1995 E420, because the dealer wanted way too much.

I did just as Phil advised...lay the new harness in place over where the old wires go. I then started at the main connection and worked my way out to the injector wires and beyond. The only issue was that a few wires at the very end of the harness could clip into place two or more ways. I couldn't believe that Mercedes would not put a notch or other feature in the connector, so that you could only put it on one way. For these instances, be sure to look at the wire color coding (stripe) or how the old connector is positioned. Only one was not obvious. The engine-mounted device had 4 equally spaced pins (forming a square) that a connector fit onto. You could have rotated the connector 90 degress and plugged it in four different ways (only one correct, I assume). When I pulled the old connector off, I looked and the connector holes were numbered. I made sure that the new connector had the same numbered holes aligning in the same manner before inserting it onto the pins.

Car runs like a champ now and I love to have saved around $1,000.