For anyone interested here is a lot more information on the Audio-6CD and HVAC unit and the problems and a possible solution to replacing the OEM Audio-6CD, especially if you are into DIY and not afraid to get your hands a little dirty.
Original Post - driveaccord.net/forums/
I decided to revisit this project and make sense of missing circuits and block diagrams.
Last time I dismantled the unit I managed to identify most of the IC’s on the main PCB. Specifically I identified the LCD Controller IC. After looking over the sockets and pin assignments on the main PCB I was starting to get really @#&^% off.
They have placed the display CPU inside the Radio. This drives the LCD data-clock, data-stream, control lines AND the key-scan controller for all audio control to and from the display Driver/Key-scan controller and LCD display circuitry located on the main HVAC PCB.
If you try to relocate the HVAC PCB, you will lose the LCD display and button functions when you unplug the radio from the HVAC board. It appeared that key-scan functions for all buttons on the HVAC PCB were built into the Display controller. If so, you may lose control of the HVAC if HVAC buttons are scanned through this device.
I discovered that an Air-conditioning data-stream is fed into the Radio unit – This is information about the current control state, settings and all diagnostic data and is decoded and then re-encoded in the Radio before being sent to the HVAC PCB and displayed on the old LCD.
Using a digital storage CRO I managed to identify a repeating data stream on this control line. Once all timing information had been calculated I set about programming a Micro-Controller to decode this stream and send raw data to the USB port on the PC. I then had the task of decoding the raw data and identifying which functions each byte or bit controlled or indicated. This has been accomplished and the picture shown is an actual screen capture of the basic display software I wrote running on the PC. - This has not had HVAC controls added at this time
In Short – The radio and CD can be removed and all HVAC functions can be retained.