just did it last week!
I happen to have just done the job last week on my 95 E320 (W124 with M104 engine). I ordered the part from Phil at the Parts Shop. According to the VIN I supplied him, I was supposed to have a certain part number on my harness. After I received it, I was about to put it in and had it all laid out along the old harness. Right off the bat I noticed one connector was different. As I went further down the harness, I saw some other discrepancies. After calling Phil and giving him the part number off my old harness, the replacement I had would not work. I sent back the old one and received the replacement which matched part numbers.
I just pretty much started at the large connector behind the battery and went one by one from there. It was actually pretty easy. Not wanting to be too hasty, I tried to make certain of the lengths of the wires, connector types, etc. before unplugging the old one. In my engine, there were 2-3 connectors that I had to do from underneath with my wife guiding my hand to the correct wire/connector but even those were not that bad. Overall, it took me about 3+ hours.
I happened to take a few pictures while I was doing it so I will try to post them here. (We'll have to see if I'm successful.) Here is a picture of the new harness. The large connector on the right is the one that plugs in behind the battery. The rubber-reinforced section plugs into the fuel injectors and plugs.
1995 E320 Wagon
1996 Volvo 850GLT