McLaren,
What you're looking for is the headlight lense housing. Cost $80 here in FastLane (under Body Electrical cat.)
Edited: before buying a new one...are you sure you seated the bulb right? Anything that's broken? if yes, the lense housing or the fastener?
AFAIK, removing the front seats in the 560SEL is not (if any) different from the rest mechanically; maybe a few more plugs to pull...
Frank.