I don't remember it being isolated in it's own sleeve, then again there could be a difference between the two.
frankly, it's a process of elimination. If the other wires are all taken that must be the one. I would imagine the AM reception is nonexistent at the moment as well?
IIRC the power antenna and the amp feed off the same turn on circuit.
Try this link- it should be blue / white- on my car i'm pretty sure mine is. This is for an E class but should be the same.
1998 Mercedes E-Class Stereo Wiring Information
rjp