I did a search and found some info on this but not the answer I was looking for. Here's what's going on. I have a 2001 GMC Yukon XL without bose, w/Onstar. Hooked up an Alpine 9884 head with a standard wiring harness and there was no sound. Realizing at that point I need the MET-GMOS 04 module to hook up to make it all work. Pulled the Alpine out and hooked back up the stock head until my module comes in. Now I have no sound! The stock head unit powers up, all presets, time, bla bla bla, works but no sound. Has anyone run into this? I'm just crossing my fingers that for some reason the Onstar has the speakers muted or something and when I get my module, it will be fine. I was just curious. I thought it was odd to just unplug the stock unit and then plug it all back in and now no sound. Like I said, I was just curious if anyone has run into this. Much aprreciated.