I did something similar to a 5th gen camaro. It was much easier. Take out the cell modem unit, open the case, pull out a physical hardware pin bridge that connected it to the OnStar unit. Done.

The only thing I lost was an in-dash compass heading.

The modem was 2g so its dead now anyway, I suppose I could put the bridge back and get the compass. I won't though, that takes effort and road signs have directions on them. There is always the sun too.