Moved to a more correct area for this.
I am not sure exactly which features are fully lost when replacing a factory HU, in an 04+, but I would say the computer, PCM, would sync up with the HU to correct for the right date and time, the PCM will store the date and time until you disconnect power from the car, just like any other and it will reset to a default date and time set by the maker of the HU. Once you set the clock's date and time it should sync up.
As far as radio having control over the airbags? I don't see how it could or why they would hook such an important feature up through the factory HU when there is the possibility of people replacing their HU's. That would be a dangerous combination for sure. Mostly the things lost in any GP with Onstar built into the HU, you lose the chimes and the ability to have cellphone function work through your speakers of your car.
When I took out my 2003's factory HU with Onstar, I only lost the cellphone portion and several chimes I lost, that really had no major impact on my car. I dont know if the airbags thing is someone's scare tactic to make you wanna buy a $150 wiring harness to retain those "special" features or what.
Sorry I can't be much more help than that.
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