I would pull out the antenna, open it, and lubricate everything, and change the mast if its dirty and old and can't be cleaned up and lubricated.

It could be a loose wire, so check all the connections.

Perhaps the wire at the antenna or back of the radio for the antenna is loose.

Alon.

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'92 300CE - Sold
2004 C240 - 744 - C7 Wheels - Android Radio
2002 C320 - 816 - Sport Wagon - Sold
2006 E350 Wagon - 368 - Dead
2015 GL350 Bluetec - 197U