The JVC stereo that the Commodore came with doesn't support Bluetooth - it has an optional bluetooth module that turns out to not be installed. There's a microphone on the A-pillar, what the heck is that for? The JVC wasn't terribly well installed, so I slid it out to have a peek behind it and couldn't see anything - the socket for the BT adaptor was definitely empty and I couldn't find any wires anywhere.
So this morning I set about swapping another head unit in - I actually started installing it Friday but didn't get it finished. Getting to the sides of the 2DIN housing turns out to be surprisingly involved in a VT Berlina, as a far amount of the dash fascia has to come off, as well as the centre console has to come out the way! While I had it all apart I figured I'd so some clean up and possibly wire an extra power socket in (which I didn't end up getting done).
Once I had the entire thing apart, I found the remains of a relic... a Nokia CARK-126 set with an HFU-4 connected to - surprise, surprise - the microphone on the A-pillar. With a tiny bit of modification, the microphone plug from the HFU-4 fit directly in to the microphone socket of the new head unit, which saved me the considerable work of running it's own microphone to a suitable location.
After several hours worth of work, I am satisfied that the head unit is installed correctly - except I pushed it about 2mm too far into the 2DIN enclosure and now the surround for the head unit won't stay on. I am not taking the entire dash apart to rectify that problem, so I may just epoxy the fucking thing on.
The new head-unit is a Pioneer FH-X775BT - basically the cheapest head unit I could find on the day that supported multiple connected BT devices (so Sabriena could play music over BT Audio, and I could still answer a phone call while driving). I had wanted to pick up an Android Auto head unit, but they're still obscenely expensive, so it didn't make any sense to go with anything too expensive when I'll want a new head unit in a couple years when they come down in price anyway.
I also bought a couple new front speakers to go with it, because they sounded very muddy... but I think it was just something to do with the old head unit as the speakers that are in it sound great now. There are marks where the door skins have been off, so I'm thinking it possibly has aftermarket speakers in it already - though I haven't had the door skins apart to check that. I'll probably just return the new speakers (didn't even open the box) because at the moment I have no complaints.