Update,
In my 2004 service manual(s) I found the following note related to the amp and the ACC in Volume 2 on page 11-85.
" Radio Amplifier
The amplifier receives audible warning commands via a class 2 message from the BCM. The chime alerts the vehicle driver to certain ACC vehicle conditions. The audible chime accompanies a visual indicator as a warning to the driver. The ACC system will not operate if the radio amplifier fails."
I have removed the factory amp from my 2004 and investigated the "quality install" inside the passenger kick panel of my 2006. They left me the small C3 connector that lists the Class 2 Serial Data, so I am going to plug in that connection, possibly cut and splice in the 8 pin C1 connector from my 2004 that has the power and ground, and see what happens (pages 11-105).
From what I can see, all I need is to power on the factory amp and I have the data connection (thankfully). I may change my mind and connect the power and ground wires right into the pins on the amp without cutting out the C1 connector from the 2004.
Based on the service manual, it seems to be the amp that matters, not the entire radio.
In another separate post, I am chasing a problem with my ACC not working after a stereo upgrade. This thread is more specific to the amp. I will update in a day or two, hopefully the ACC decides to work after it "receives signals" from the factory amp again.
FTH