Strap wrench worked perfectly for me:

http://www.sears.com/shc/s/p_10153_12605_00945533000P?keyword=00945533000&sLevel=0

I messed around with that bolt forever, but once I went out and bought the wrench it was a 10 second job and it unbolted right away. Don't hack up your hands trying to get that bolt out.. just grab a strap wrench and you'll be done quickly!

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'83 300SD 335,000km (207k) mi SOLD
'87 560SL 163,000km (101k mi) SOLD
'86 300SDL 356,000km (220k mi) SOLD
'92 500SEL 250,000km (155k mi) SOLD
'90 300SL 140,000km (87k mi) SOLD
'01 S430 260,000km (161k mi) SOLD
'03 SL500 167,000km (104k mi) SOLD
'07 S550 4MATIC 235,000km (146k mi) SOLD
'07 GL320 CDI 4MATIC 348,000km (215k mi)
'13 GL350 BlueTec 4MATIC 170,000km (105k mi)
'14 SL550 72,000km (43k mi)