Really interesting answers. Thanks.
However, I decided to use the old Garmin to free up my phone despite a rare outing with the Rivetess as co-pilot. (Mostly in command) Anyway, I footled for ages trying to get the Blue Tooth sound into the Sony radio. Failed totally. Then I tried my phone back with the radio. Suffice it to say, I spent the next hour footling about again with no success at all despite it having worked for in that little car for over 4 years.
Delving into the mystery will have to wait - being taken to Bletchley Park for my 80th Birthday present. I've just looked at it on G-Earth. Ewwwwwwww, is there any space that humans haven't built up?