The adapter you need for the rear amped audi system is the PC9-401 Standard System The standard concert headunit runs only the front speakers, the rear speakers are amped up and so is the sub. they run at the following powers: Front 2 x 15W straight of the head unit Rear 2 x 25W off the amp in the rear Sub 1 x 80W off the amp in the rear by upgrading the head unit (probably 4 x 50W) you have had the following effect: Front 2 x 50W straight off the head unit Rear 2 x 50W which then runs to the Audi amp and reduced to 2 x 25W Sub 1 x 80W off the amp in the rear The factory amp is lowering the signal to the speakers which is where the well known interference comes from. What you need to do 1/ remove the plastic inner sills that run down the length of the car 2/ remove the lower part of the back bench so you can pull the carpet up 3/ pull the carpet back to reveal the wiring looms near both rear doors 4/ Find the loom thats got two wires taped up (its held down with duct tape) 5/ unplug the Audi amp in the boot and turn your stereo off. 6/ Cut these two wires to each door. 7/ run new speaker wires from the rear outputs on the headunit along the sills 8/ connect the new wires into the wires you have just cut coming out of the doors 9/ tape up the old wires coming from the Audi amp 10/ plug in the audi amp that now just runs the sub 11/ replace all the trim/carept and seats 12/ enjoy your stereo that now runs at 4 x 50W with an 80W sub.