It sounds more like the mic on the radio is either blocked (if it's internal - which is a terrible design, by the way) or not installed properly if it's an external mic. It should be installed so that you can see it. (I have mine running from under the dash, up the front-left quarter panel, then across the top of the sun visor stuck on with Tac 'N Stik [with a "service loop between the quarter panel and the sun visor), then clipped onto the edge of the sun visor facing me. No one ever knows that I'm on the car audio. [Back in the day, when mirrors were fixed to the roof, we used to put a telescoping arm on a ball joint above the mirror to hold the mic for MTS mobile radios - same idea - if the mic can't see you it has a problem hearing you.] If it's internal to the radio, which a lot of them do, if there's no way to plug in an external mic, there's not much you can do but bring it to your GMC dealer and complain that the design itself is a factory defect.)