I said it wouldn`t work, I didn`t however say it was impossible.
But I stand by my comments - the CD70 is a crap stereo, and the audio processing is nothing better than the NCDCXXXX range of stereos that integrate so easily with the Omega. You want MP3 or DAB, then use a Parrot kit or Pure Highway through a modulator - thats easy, but don`t try telling me that the CD70 will work fully in the omega, it won`t, nor is it worth the hassle. And have a close look at the Cavalier you mention, I know it well, and there are more than just a few functions that don`t work correctly - All that expense of ecu`s and modules, and it still does not and will not ever work correctly.
We put a Pioneer AVIC Navigation on a shopping trolley once as a P.R. gimmick, sure it worked and even got a speed pulse from the front wheel - got us a great feature in a magazine - was it worth the effort and expense when you could clip a Tom Tom to the handlebar? Of course not.
There are easier and better ways to go.
I seem to recall that some of the older top end Alpine systems used to have a DIN sized commander module that outputted its functionality to an external monitor - the system was expensive, but probably no more so than the time and effort wasted with Vauxhall ecu`s.