To add some extra information;

The Becker 780 is original fully function, with the FM/AM and cassette.

I have added an Bluetooth module and an IR sensor for the remote.
In radio mode you can use the remote for the volume and changing the station.

When an phone is connected to the Bluetooth module, it will switch to to the audio of the phone, and will start an mp3 (streaming)
With the remote you can do Next/Prev, Play/Pause number, volume ofcourse, Take or reject a incoming phone.
You can also use the cassette buttons for this.

You can switch the radio back to FM/AM if you want to listen to the radio.

If your listing to the radio (FM/AM or cassette) and you get an incoming call the radio switching automatically to the bluetooth, and you can hear the ringtone. Then its up to you to answer or reject the phone.
If you reject or your done talking to the phone, the radio is switching back to the FM/AM.
Or your where listening the streaming MP3 it will resume that.

There will be an external MIC on it for the speaking part.

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1986 W201 190D 2.5 ~ From bare bone to almost full option.
1985 W201 190E 2.0 ~ Project car