I just bought a used 03 GMC sierra and something isn't right with the stereo system. It works but is not very loud. When it is turned all the way up it is about as loud as a normal talking voice. The head unit is a tape with single cd and does not say bose on it, but the door speakers do and I can see part of an amp underneath the center console when I remove the cup holder. I can't find anything online about what could be wrong. Hopefully someone can help.