If you're addressing the Audio Gateway, it is located in different places in different cars - and even within the same class, it may be integrated with the head unit (as it was from '00 through '02 COMANDs) or it may be separate. I wasn't successful finding the post, but buried in memory I have a recollection that the AGW in the '03 and later S-Class is under the right rear passenger seat. The seat bench is very easy to remove - so don't shoot me if my recollection isn't correct.
If you're addressing the CAN bus, I don't know its location in the S-Class - but its connector block is located in the footwell in front of the driver's seat.
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2005 S500 4-Matic
with Gateway 500 iPod integration
Formerly: 2000 S500
with Ice>Link iPod integration
Moto V710 phone with MikBox
Upgraded Voice Control
Its for my friends 2001 S500. He had the car diagnosed as having a faulty CENTRAL GATEWAY, A2204451900
in photo below.
He has the part but MB dont know where it goes !!!