How do you replace an antenna on a Mercedes 300E?

I'm guessing you mean the antenna mast. Here is an excerpt from the manual.

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Antenna Mast: Service and Repair

Removing and installing telescope of automatic antenna

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Removal

1. Unscrew securing nut

2. Switch radio on (2nd person required).

3. Withdraw the extending telescope (2) with the plastic cable (3).

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Installing

1. Insert plastic cable with the teeth in opposite direction of travel (in direction of travel for Station Wagon).

2. Switch radio off (2nd person required).

3. Allow plastic cable and telescope to retract fully.

4. Screw on securing nut.

Hope this helps.