X2!
It's so simple...^^
...if you had this port on your radio.
Most of the old radios have tham. Mono-radios with 6 pols and stereo with 8.
I only know who to make by the mono-radios...
All you need is the plug for that port, 3 cables and simple socket for the AUX-IN.
One cable you connect with the ground-cable from the radio itself. Than you connect one cable with pol number 1 from the DIN-plug, the last cable you put on pol no. 2.
Now you are happy that the sockets are so practical build. It has three pols: One is near the hole where the aux-plug came in - thats the ground, so put the ground-cable to that pol. And than there are the two other pols at the end of the socket. One goes to a little plate which was pressed by the side, when you put in a aux-plug. That one is for the cable, which is connected to pol no. 2. And now there is only one cable and one pol left. So connect both and you are ready.
Now you can listen to the radio, and if there was only shity music or you just want to listen to you own music, just put in the plug and you will here the music from your mp3-player or laptop or what ever you connect with. If you remove the plug from the aux-socket you can listen to the radio again.
Simple but great
I haven't brazed the stuff yet. I'll make some pics if I do it.