Have you considered adding a Fuba-style whip antenna from a late-model VW? You'd have to lower the headliner and drill a hole in the roof but these antennas are cheap, have great reception and are very robust. When I took the audio system out of my Miata I found that there is a factory rubber plug for the antenna hole on the rear fender -- perhaps Mercedes makes a similar part?

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1983 240D automatic, Orient Red, 174K
1983 240D manual, Biscayne Blue, sold at 341K