Okay, all sorted now.
I found this thread to be the most useful in getting it all to work.
https://www.fiatforum.com/i-c-e/3374...ml#post4280527
See post
#16 for the detail. Many thanks to davet2001.
Complete solution was as follows;
1. Note that most cables you can buy only come with 3 pins populated and will result in the 'no source available' message. You need to add a link between pins 13 & 14 to fix this. See picture 3 in the above link.
2. I resolved the issue with the phones not detecting the cable/stereo by shorting the left channel and ground pins on the connector block using a resistor. It's all to do with the android standards for headphone detection.
A simple piece of wire on these pins will allow audio through but the left channel is quieter than the right due to the signal bleeding back through the unrestricted link. I tried a variety of resistors pushed into the block in order to get to a point where the cable was detected but audio unaffected. A 470K ohm resistor did the trick.
The pins are 18 & 19 or the black & white cables in picture 3 in the above link.
I removed the pins from the ISO connector using two paper staples with one leg straightened. These can be pushed into the bottom of the connector to release the two small tangs that hold the pin in place and then push it through with a small, flat blade screwdriver. This is far safer than attempting to pull them out through the top with pliers.
Cables can then be soldered and replaced.
3. The cable can be routed either out the back left of the radio cavity and into the passenger footwell (you can see the gap if you peer in) or down through a hole front & centre of the cavity just behind the hazard light button.
I did the latter & mounted the 3.5mm socket into one of the unused blank buttons above the gear lever.
The hardest part was removing the insides of the button in order to fit the socket. A small saw & dremel router did the job once the hole was drilled.
Now I can annoy everyone with a selection of banging tunes. :slayer: