Here are my two cents for a Becker Europa 2 Stereo radio with 5 pin DIN aux for normally a cassette deck.
Here is the original topic (where even some of you have commented on)
https://www.vintage-radio.net/forum/...ad.php?t=41176
In the enclosed schematic you can add double poled switch to choose between the radio input and the iPhone/ iPod/ … input.
You can even go further and charge your device with pins 2 (-) and 7 (+), but be carefull that your radio can deliver a decent current to it.
I use it to switch an audio relay via a switch, because I had a noise problem with the switch…