On sedans the wire goes under the passenger side floor mat - may be the same on wagons. I don't think there is an old and new air mass meter - but there is two different engines - 4matic has 103 motor with air mass meter, and RWD has 104 motor with air mass sensor. If it's a RWD 300TE the air mass sensor replacement is essentially self explanatory - take out old one, put in new one.