Had same thing happen to me!
My '94 E320 had the AM/FM antenna and the telephone all in the main antenna (the one that goes up and down!).
But on my '94 S500, the FM antenna is in the bumper but the AM antenna is in the back window. The FM reception was great but AM was almost non existant!
Just so happens that the back window had metalized tint put on and it took a lot of time before I realized that it was the metal tint in the film. Yes, I thought that it was the antenna cable, etc. but it was none of that.
Rather than take it off, I changed all connections to be like the '94 E320; ie all in the antenna. Went to a audio/radio store and the techs added an switchover relay/amp. Any quality audio/radio shop can do it for apx $250.
Regards . . . . JimF
'94 S500 Cpe
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