I am curious what still gets displayed in the radio display when someone upgrades to an aftermarket head (double din with metra kit for example). Does the HVAC, Clock, and Ambient still work correctly? Does the radio display stay blank or display an error of some sort? Has anyone ever reverse engineered the link between the radio and display so that maybe an external controller could be created to display other information?

My project RX8 has a nav display which I will likely replace with a tablet so I really don't need a double din display in the Radio but I would really like to have Blue Tooth integration with the radio so I may go with the double din radio just to get that feature.

 

The GoodBox is one such device to change what the center display shows. It can show OBD information like coolant temp, AFR ,fuel trims, etc.

No idea about the bulk of your question though.

 

The Metra kit allows you to retain the normal temp/time and add your own custom text.