2004 Buick Rendezvous Amplifier Location?

A friend of my girlfriend has this vehicle. Her audio often drops out for days at a time although the head unit (factory GM) appears to be fully functional but no audio at all. She brought ti to a local car audio shop and they told her the factory head unit went bad. They sold her a  low end Panasonic head unit and installed it. It worked fine for a few days then the audio was gone again. Same symptoms as before, head unit (Panasonic) appears to be fully functional but no audio at all. So I told her I would take a look at it.

I reinstalled the factory unit to see they symptoms myself  to make sure. As soon as i installed it, it started working perfect. So I took it back out and put the Panasonic back in, and now that works perfect too. She preferred the factory head unit so I re installed it and she was happy. The next day she calls and says it's doing it again, then a little later in the day she calls and it's working again.

I think there must be a factory amp somewhere in the Buick and it has to be causing the problem, unless the audio is somehow routed through the BCM and the BCM is telling ti to stop audio.

So my question. Anyone know where the factory amp is located on the Rendezvous? Sorry for the long read. Thanks guys.