As Harley man says MB is the best place to get it, I might be able to get you a pt. no. for the loom if required.
Once you have the loom all will become clear, from memory, the loom runs from under fuse box down into the drivers side footwell and back to just in front of the drivers seat, then it runs off to under the centre console and up to the switches. The loom will require two feeds from the fuse box, one small ign. supply and one heavy permanant live, one lighting feed which can be spliced off of one of the adjacent switches illumination feed, an earth connection to go on an existing earth point, then of course you plug in your switches and seats, it is really just plug and play, the genuine loom will have all the right crimps on already.
Do make sure you spur off of the correct side of the correct fuses though. If you look at your fuse card it will list which fuses to use. One is a small sized fuse which supplies a lot of ign. supplied items as well as the heated seats (trigger supply for switches/control unit). The other will be solely designated to the heated seats.