Audio Systems: 2004 Trailblazer, aftermarket radio, factory radio

Question
I have a 2004 trailblazer with onstar and i'm putting in an aftermarket radio, i have the gmos-04 and there is no volume control on the aftermarket radio. There is some sound coming out but not much. I tried another radio and it does the same thing. The wires are all correct. I have the amp wire hooked up and then thought maybe it wasn't getting enough power so I hooked it in with battery power still with no volume control.

Answer
the gmos-04  is made to be used in your vehicle if it comes with the bose system  and the factory amplifierneeds to stay in the vehicle and be in workng conditions,
plug the factory radio to test it also inside the interface there is a gain controls

good luck