Forget the diagram. Stick your left hand through the top left corner of the board, pull and your in. Less than a minute job. Just remember to pull hard.
True; but (using the excellent info here while investigating my new-to-me 2010), I found it much easier to reinstall the panel by first removing the flat floor panel. Only two cargo net anchors hold it in.
For MY '07-'09**, there were three batteries as described in preceding posts - two large batteries (a 35 AH "Starter" battery, and a 95 AH "Electrical Consumer" battery) and a small 1.2AH auxiliary battery. Apparently beginning in '10 ('09 in Europe) the W221 went to a single large "Main" battery (95 AH, 850 CCA VRLA, or AGM - absorbent glass-mat) for starting and electronics, located in the engine compartment, and the small 1.2 AH Auxiliary battery. Those models do not have the "Electrical Consumer" battery in the trunk. The small 1.2AH Auxiliary battery is under the dash panel as shown at post 5 above (note that one removes the side dashboard fuse panel only to remove a bolt. It is then necessary to remove the under-dash trim to access the battery). See posts 12 and 18, http://www.benzworld.org/forums/w221-s-class/1938154-s550-4matic-dead-consumer-battery.html
. For all models the small 1.2 AH auxiliary battery backs up the electronic transmission shifter and (possibly) other stored memory functions.
**Apparently some North American MY '09's were produced with only two batteries, as the '10 and later were. I do not know the date of changeover, but see posts 28 through 30 below.
Wampa007 has provided info on the specific sizes of the three batteries used in the North American MY '07-'09 HERE
Models with ECO start/stop will have an "ECO start/stop" battery in the trunk (photo, post 5, http://www.benzworld.org/forums/w221-s-class/1938154-s550-4matic-dead-consumer-battery.html
). As of MY 2011, ECO Start/Stop is standard on the S350 BlueTec models in some countries. See here
(Note that seubill points out below that ECO Start/Stop was not made available for the W221 in North America).
In the photos of my car below, the upper left one shows the whole area behind the back seat. Below the white storage compartment is the rear SAM together with the fuse and relay module. To the left of it (second photo) is the Tele-Aid (MBRACE) module; the item in the rubber case is likely the pump for the drive dynamic seats. To the right of the SAM (last photo) is a pair of empty shelves.