Keith, if you are talking the small one, located in the back of the trunk on passenger's side, up there in the top rear corner - the tank has two studs, top and bottom, about 1cm thick. There are clips that fit on those studs and that are then anchored in the slots in the cardboard side panel of the trunk. Look for cut out slots in that panel in the top rear corner. You fit the clips on the studs of the tank and then you fit the tank on that panel so that clips slide into the slots. It is difficult to get them back out if needed...
There is a third clip a bit below that holds the three lines coming out of the tank. There are slots for it in the panel too.
On the picture you have: 61 is the 2 clips holding tank. 68 is the clip holding lines. I bought both from Mercedes about a year ago. Some of the clips for wiring harness can also hold this tank.
A lot of info about it on the Forum. Search "fuel expansion tank".
