The 3-pin plug you have pictured in posts #7 & 8 is one of the two you need. There should be another 4-pin plug in there somewhere. What's that connector in the back of your picture with the blue piece on it?
Once you find the 4-pin plug, those two plugs are where the
COR-HAR cable plugs into. Then connect the
GM10-Aux to the COR-HAR and you'll have RCA inputs to your stock HU.
You can run the cable under the trim/carpet to the back if you want to, but there is plenty of room above the passengers feet if you'd prefer to just tuck it up under the front dash, your choice.
I'm guessing that you have the selective ride control system in your car because the "amp" you found is actually the control system for it.
From GMpartsDirect.com:
GM PART # 10302714
CATEGORY: Electronic Control Suspension Hardware
PACK QTY: 1 CORE CHARGE: $0.00
GM LIST: $809.18
OUR PRICE: $461.23
DESCRIPTION: MODULE
If you have any more questions just ask away