For the OEM deck, an add-on unit like this is perfect:
https://pac-audio.com/catalog/auxili...tions/aai-gm12
I ran a similar model in my '03 Yukon, which installed as a pass-thru to the XM radio unit behind the glove box. It's been a number of years now, but I was always impressed with the product quality and service expertise from PAC Audio.