I had exactly the same problem on my 8v. the door speakers werent working. so I pulled the HU out and unplugged it and checked the wires going from the dash to the back with a multimeter for resistance.
the Problem was that some numpty spliced the wires for the door speakers into the wires going to the rear speakers, which increased the resistance in the wires so there wasnt enough power for the mb quat door speakers. what I did to solve this, I diconnected the sh!tty dash speakers and feed only the door speaker with the wires from the dash speakers. then repaired the cut wires for the rears and its working very well now and finally I can turn the radio up loud without the sh!t dash speakers crackling away. hope this helps.