I intend to install an fm modulator. I cannot find the antenna cable anywhere underneath the dash, nor beneath the passenger side dash. Where is it located?

No Its not. This is my 4th benz diesel and know where to look. This 350sdl has a power hardness, dimming hardness, alarm and an 8 pin dim wire. No antenna to unplug.

There has got be a 'coaxial cable' comming from the antenna going to the radio! .. How it is adapted to hook up to the radio I have no idea .. but there is a coax cable going to the radio from the antenna!