in-dash changer in an '03 silverado

I got a good deal on a Delphi factory in-dash 6 cd-changer ('02 part #15179349) and am trying to get it to work in an 2003 Silverado that had a single CD/cassette radio. While the grey plug is the same and goes right in, I can't get any sound out of it. It powers up (doesn't say "LOC" in the display), and can do all the funtions, but I can't get any sounds - AM/FM, CD, or the door dinger to work.

I was told when I bought it that it would have to be activated by a GM dealership, but when calling several service departments, none of them know what I am talking about. They said that as long as it didn't say "LOC" in the display at start up it wasn't locked and nothing should have to be done. Does anyone know more about this and what would need to be done to make it work? Any and all help is appreciated!!