Update:
I took out the constant 12v wire in the "Audio unit relay holder" and swapped over the redundant Ignition 12v wire from the "Rear seat heating relay holder" to where the constant 12v wire was.
I can now have the radio playing when the ignition is on! time will tell if this has also stopped the battery drain as well (will know in a day or two) from fuse #72?
It was pitch dark outside when I finished so I couldn't be bothered taking the spare wheel etc out to do an inline measure with the multimeter.