Jason,
I had an installer add the rear speakers along with a Blaupunkt head unit (had to replace 2 Blaupunkt units due to the motorized face plate break down) and he ran the rear speakers off the RCA outputs (I have a 2004 coupe ltd) so when I installed the Pioneer I ran the rear speakers off the RCA outputs.
I checked the wiring today and rewired the front speakers from the Pioneer white/white-black and gray/gray-black to the VW harness wires same wire colors and the volume is great in the front (much more volume even at a low setting). Hooked the rear speakers green/green-black and violet/violet-black from the Pioneer to the black/red and black/white from the rear speakers and the volume is not as loud as the front, am I doing something wrong? should the front and back speakers have the same volume?