I guess no one wish to comment on a circuit which consists of unknown parts
Fully understandable. I took the risk and all seems to work well.
Anyway, I did cut the PCB traces before ICx3 and soldered a female DIN lead here and two capacitors for blocking DC. I fitted a male DIN lead to a audio jack that I installed in the car's dashpanel. I could have skipped the DIN connector and wired the jack directly to the radio, but this way it is easier to unplug the radio if I have to take it out for some reason.
The jack is switched so if no plug is inserted the signal passes from the radio/cassette circuit, to the jack and back in again, to the amplifier stage. When a plug is inserted the internal signal is disconnected and the auxillary signal is routed to the amplifier stage.
I had a concern that the disconnected signal would crosstalk to the other wires in the cable since they are not individually shielded, but that was not the case.