Speaker wiring

[UKenGB]

Still trying to get to the bottom of this.

My UK 2005 has basic audio which means 6 speakers (2 x dash, 2 x Fr. doors and 2 x Rr. doors) with Nav/screen etc up front and radio/amp down beside the battery (I think).

However, other variants had 4 speakers in each door. As far as I can tell, my front doors have wiring there for the tweeters. But...

Do the rear doors also have wiring to them for the other speakers not installed in my car?

If so, from where do these wires start? Do they all come from the amp area (although not used currently of course), so crossovers could be installed in the boot and feed 4 speakers in each door without any additional wiring? Or does the BMW system expect crossovers installed at some other location with only 4 channels emanating from the amp (when installed)?

I'm trying to get to the bottom of this before ripping the car apart and getting stuck while I figure out wiring complexities, with the car off the road in the meantime. For which reason, I really want to know what wiring exists and where it is. Once I have this info, I can plan how I utilise it to my best advantage.

So hope someone can clarify this.