Audio adventures on a 6 - What do audio Ref numbers signify?
 

I'm trying to get a working audio head unit into my 2007 Mazda 6. The one that has come with it will only play 2 of the CDs I own for no reason I can discover. I've cleaned the laser, still it will always play the same two, never any others. One is a copy of a full commercial disk, the original of which the player will not play. I've given up on it, it's too eccentric.

I got hold of a Multi-disc/ cd changer head like the one I had in my old TS2. It went in fine, all the buttons worked etc. But the sound is very quiet and distorted. Obviously a compatibility problem.

So for the next replacement (I'm buying these on eBay) I'm being more careful about my selection. The player that came with the car is model no. GR4B66DSX, ref no. CQ-MM45701A. I notice that many of those on sale do not give a model number but the ref. number CQ-MM4570AK is very common. Does anyone have an interpretation of what these ref numbers mean? Are these two numbers compatible units?

Alternatively, can anyone point me at an aftermarket head unit I can swap in, with a face plate adaptor available? The model fitted only has two connectors on the back, one of which is not connected (believe it is for an external cd changer, but is sometimes used to connect a USB/ aux unit).

Many thanks for any help you can give me.