Where do I plug this wire?

The main part of the lead goes inbetween the aerial and the headunit, but where am I supposed to plug in the little blue wire?
Basically, to some up my problem and my assumption, I'm getting poor radio reception with my new headunit, so I'm presuming it's because there's a powered aerial in the rear glass that is no longer receiving power and hence I need to re-supply it. Is this right and how do I do it?

The link also says this is for VW, Audi, Seat etc. I thought you had an MB?
On a 2004 C-class you should have a four way diversity antenna at the rear screen. The selection of the antenna is done with IF quality detector at the head unit. I have not looked at this but I assume this signal is fed through the antenna line while the antenna amp unit would be powered directly form a different wire. You would lose the diversity feature anyway but you might be able to fool the MB amp to use constantly one of the antennas with a proper feed. I would have measured the control voltage from the OEM radio (actually I would never have gone for an after market route, but that is a different story).
I bought a cheap external aerial, plugged it in and then stuck it out of the window and it worked great! I guess I need to work out how to use a new aerial instead of the one built into the rear glass.
I might go down to an auto electrical place and see what they say.

The point being that if it is a new MOST based car, it should have the rear FM antenna powered directly from the rear SAM, then it should also give signal unless the wire is broken. Another scenario would be that the rear SAM "knows" from CAN bus that the radio is off and does not power the antenna.
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The main part of the lead goes inbetween the aerial and the headunit, but where am I supposed to plug in the little blue wire?
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