I guess my thinking is too simple. My electronic abilities end with replacing batteries.

1) If plugging something in the jack opens or closes a switch, I can use that switch as a power switch. I might need a relay if the switch opens when the jack is in use. I only insist because the last thing any car dash needs is another switch. However, I see the case of wanting to listen to the radio while a device is physically plugged into the jack. And you might have the jack out of reach such as in the glove box.

2) I still think the antenna has little to do with it. Any car radio from the last few decades will do a fair job of receiving an FM signal with no antenna. Using the FM converter to physically disconnect the antenna from the radio, which I think all such units do, will not eliminate the radio's ability to receive FM signals. What the device does is give it's wired transmitter a fighting chance.

Gilly once helped me obtain the radio code for my 91 300SE. When the dealer gave me the wrong code despite verifying the right s/n for the VIN. Is Gilly still around? If not, I think the folks at La Jolla auto sound who run the Becker forum can help with codes.

Sixto
87 300D