ok, it took me alittle while to get it working right, but i finally got it. :dancing6:
i successfully installed a 2004 CD6 in my 98gt. 01-03 CD6 are different than the 2004+ CD6.
basically, all you have to do is get 3 metra harnesses.
1 to plug into the car's original harness
1 to plug into the radio to power it up
and 1 to plug into the radio for the rear amps.
then, to get the proper signal to the rear amps, you have to bypass the front amp, all you do is connect the "AMP OUT" harness to the factory harness that leads to the rear amps.
if anyone wants better details on this, let me know and ill get more indepth on it. the same method will work for all CD6 units out there, im sure the NAVI systems are the same, i just havent had a chance to look at them yet.