9 Speakers + Aftermarket Reciever?

[99gst_racer]

The vehicle is a 2001 Suburban LT.

 

I'm not at all familiar with how everything is wired and configured. Is there a factory amp somewhere? I assume the factory receiver doesn't have outputs for 9 speakers, so how is it wired to allow a basic 4-channel aftermarket receiver to be used?

[cgalford]

The factory amp has a 4 ch input and using the proper interface kit will make it work with any aftermarket stereo.

[99gst_racer]

Where is the factory amp located?