Right, an update.... I removed the CD changer from glove box and checked the wiring, nothing to report there. I then moved onto the head unit itself, removed that and pulled the harness out so I could have a look at plugs, and quadlock etc to ensure that there were no wiring issues and the CD changer burst into life. It lit up and started making noises. So, obviously a wiring issue between the head unit and the changer.
I put a CD in the changer and selected it on the head unit to play, it didn't play. It just kept defaulting back to the DVD player. When I checked the head unit display it wasn't actually showing that it had an audio CD in the changer, just that there was something loaded into it. So the head unit looks like it's not receiving the data signal from the changer to tell it what is loaded into it.
I checked the quadlock, and there's a lonesome yellow wire about 2" long which looks broken (I'll post a picture when I next go out to the car) it looks like it should be connected to something, but it's not. And at this stage I can't see where it should go as I can't see any other yellow wire around that area.
Like I say.... I'll post a photo and maybe someone could make suggestions regarding this wire. I've checked the online pinout diagrams, and they don't colour code the wires so it's hard to identify what this wire actually is.