I appreciate that google could have been my friend, but I wasn't confident enough about variations etc. Especially given that both the car engineer and his supplier had thrown their hands up.
Thank you very much for your assistance.
I have a DAB digital radio box which can be wired in such that the car antenna cord enters the box through a DIN-in and the DIN-out goes to the AM/FM antenna input on the car audio system.
However after speaking to a car electricals engineer I learnt that the car antenna would not have a DIN...