OK, a bit of background. I re-installed my re-furbished Hirschmann 6000EL in my 380SE, and moved the aftermarket unit that was in the 380 to my Mazda 929. The aftermarket unit has a screw-in fitting that the Mercedes coax audio lead screwed right into. On the Mazda, I needed an adapter to plug into the standard audio lead socket. The unit did include one of these, but I couldn't get any continuity in the inside wire. I put it in anyway, but got very poor reception. So, I took it out and took the plug end off. Inside there was a tiny cylindrical brown thing, marked with '221', soldered just inside the pin of the plug (resistor, capacitor??). No continuity through this thing, but the wire itself was fine. I removed it, resoldered the plug, tested for continuity (OK) and put it all back in. Now I get good reception.
My question: What the heck is it and what is it for?

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1986 560SL
2002 Toyota Camry
1993 Lexus