Done a bit more on the head unit today. Went to town with the dremel to get the surround built up. Fitted it in the car using a multi meter to work out what was the live for the bose amp and wired that into the ignition live.
Finished when it was pretty dark and there is still some alignment to do but this is what I have so far.
It was an absolute nightmare and my hands are cut to pieces, the access is bad, you have no space to work in, routing wires is a pain and the climate wires are only just long enough. It must have taken 30 mins to get them plugged into the back of the climate control.
Couple of issues, the alarm is now firing a single bleep when i lock the doors. This is a warning that something is wrong, there is a circuit on the old casing which is insulated and grounded on the old unit shown below:
It looks like I just need to insulate the wire to prevent the issue here so I have disarmed the alarm for the night. Went out for a drive and for a couple of hundred quid the unit is really not that bad, sound is about the same as it was before and it has plenty of features so happy days.
The only other issue is that I need a converter for the radio input but at £4.99 from halfords it’s not such a big deal and I will pick one up tomorrow.