If this model is similar to the Pioneer DEH-P3300 that I just removed from
my own car,
http://www.pioneerelectronics.com/PU...yers/DEH-P3300,
which it looks like it is, it should have a feature that will let you use
the rear speaker output as either normal rear speakers or a subwoofer
output. Although I never tried this feature myself, when it is in the
subwoofer mode, it may not let you fade to the rear speakers because the
head unit thinks that you have a front speaker and subwoofer setup. To
check this, power off the head unit with the key turned to accessory, and
hold down the Function button for 4 seconds. A menu will display on the
screen and one of the options in this menu lets you switch between rear and
subwoofer mode. Changing it to Front/Rear should fix your problem.
Not sure that this is your problem, but worth checking out.
Sharky