AUX IN is just for sound, not control. Also, the AUX in gets processed through the BM53.

Need to confirm OEM speaker wires are intact, which you can do by removing plug from BM54, then using an AA battery, apply 1.5v to each speaker wire pair in turn. You should hear a small click from the speaker as you apply (and remove) the 1.5v.

Or use a multi-meter.. :-)

BM54 pin-out is...

https://www.bmwslo.com/uploads/month...1466425700.jpg

The spare connector could be for the tow bar module.

In my opinion - potential for failed bm54 is very low especially considering the fact that those chopped cables are definitely aftermarket and 99.9% likely to have been speaker related. As I said before, the black cable containing 8 wires looks highly suspect to me.

It sounds silly but first I would check you actually have speakers in the door pods. If aftermarket speakers were fitted they may have been stripped out before selling like it seems an aftermarket amp has been removed.

Also, if you know for sure there are speakers then work back you know you can get them working again. All the troubleshooting in the world wont make speakers work if there are non there in the first place!

A quick and easy way to check is holding a bright torch up to the speaker grille on a door panel- you should see a speaker cone behind the grille.

If your X has speakers, you can get them working again - you just need to investigate the wiring. My guess is the original factory speaker wiring has been modded on your X and not put back to original when the aftermarket amp was removed.