Have you looked behind the driver's side headlight assembly? That's where mine is located. It looks like a round tubular assembly with a small (1/2" dia) glass on top.
BTW, MB specs says you should hold the engine at 2000rpm for that test! Also my engine temp was 85C and I did NOT do it at 2000prm but at idle.
Regards . . . . JimF
'94 S500 Cpe
Visit my Mercedes Web Page