Mondeo Mk4.5 2011 Sony for Audiophiles Stereo Unit Issue

HI

I have a pretty common (at least online) issue with the stereo unit (or not) and I cannot seem to be able to trace the fault due to lack of electrical information that I cannot seem to find anywhere and I would appreciate some suggestions.

Here's the issue:

Like other people, I experienced the random, occasional "no-sound-but-crackling" issue until recently when the sound hasn't come back for a few weeks. But this is not where this ends as I did get somewhere. 

Vehicle: Ford Mondeo MK4.5 2.0 TDCI Titanium X Estate.

Audio: Sony for Audiophiles with DAB, BT, USB, AUX, Single CD, no CD changer - 9 speakers including dashboard tweeter + the 10th Factory Subwoofer in the boot. That's as far as my counting goes. No sound amplifier by the OBD as far as I can see.

So, the audio system has 4 fuses. The passenger footwell F21, which is the power fuse for the head unit, irrelevant in this case. And 3 fuses in the boot: 

- FC5 (7.5A, supposedly for rear entertainment and CD Changer, which my car does NOT have, but the fuse is present, just as my car does NOT have a sunroof, yet the fuse for Sunroof is plugged !?)

- FC9 (20A for Audio Amplifier, unknown location !? but I understand it's built into the head unit, although why would there be a separate fuse for it?)  and 

- FC10 (10A for Sony Audio System, named like the F21). 

BUT here is the fun part. With FC10 fuse removed, the head unit works fine and outputs sound faultlessly, repeatedly and consistently, but only to the front tweeters (left right and centre/dash) and the boot-subwoofer. No sound to any rear speakers or front-mids. 

But all functions of the head unit work flawlessly, DAB, BT and literally everything.

So, basically  I get an overall very-heavy-bassy-high-pitch-scream.  But as soon as I plug the FC10 back in, the crackle reappears. Take the FC10 out, the sound returns and works from there.

Each of the speakers (25W FoMoCo) has been taken out and tested with a multimeter and all speakers show very similar values. The stereo was also tested each time a speaker was unplugged. It was tested in all possible combinations including with all but one speaker plugged. But no sound.

Each speaker has a blue capacitor (forgot the exact value), and all capacitors have been checked but none look popped or swollen.

Fuses are all good. 

Fixes:

Head-unit has had the thermal pads replaced, hoping that would cure the problem in case it was overheating.

Conclusions:

The head-unit works, the speakers appear to be fine, the wiring visually does not look damaged.

Questions:

Where does the FC10 fuse lead to or what does it power and why does the stereo work without it? Can anyone help me trace the wiring from it?

Any suggestions highly appreciated.

And I accept that I may have missed something since everything appears fine while there is clearly an issue.

I also apologise if anything is unclear, so please feel free to comment and I will clarify any misunderstanding.