I'm sure it could be done. However, there was a change in the middle of the 2004 model year production from a Class 2 amp to a non-class 2 amp.
GM now makes both LUX (Bose), and NON-LUX (non-Bose?) navigation head units. As far as I know, all 2003s have the LUX class 2 amp. So you 2004 guys have an additional twist to consider.
Here is a link to a good thread the Chevy Avalanche Forum:
http://www.chevyavalanchefanclub.com/cafcna/index.php?board=157;action=display;threadid=21297; view=all