Not sure to be honest. Like you I did the same and have the rnse unit in my garage. However I'm thinking of going the aftermarket route as I like the idea of having Bluetooth on the screen and a reverse camera possibly.
It's not easy. You'll need the head unit, upgraded antenna, wiring harnesses, additional modules?
The simplest option is to fit an aftermarket android auto/apple car play touch screen.
If it has an alarm I suspect the siren will be in the boot side panel.
Electric aerial was standard I think as was the headlights on function. Even my old ford escort has that.
If you have a sticker in your boot it might have the different codes on it which would highlight the spec when codes are put in a decoder.
Think clear lenses were fitted to facelift cars if I'm not mistaken
I've seen on the car campers groups on Facebook that people have made their own blinds. Using cardboard as a template then using thermal bubble wrap insulation from b&q, insulating roll similar to make custom ones. Plus this link might help:
https://ethanmaurice.com/blog/how-to-make-blackout-window-shades-for-a-camper-van-or-element
It's a rubber compound. You can buy stickers that go over the existing switch gear like this:
https://www.ebay.co.uk/itm/394651879019?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=avueyyaerko&sssrc=4429486&ssuid=IE07kO4STZy&var=&widget_ver=artemis&media=COPY
A bit of a random one, are all the jubilee clips holding all the pipes on around the turbo etc tight?
I ask as under pressure air could be escaping from I'll fitted pipes that wouldnt be visible when idling if that makes sense
Something like this?
https://www.ebay.co.uk/itm/384772391273?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=J1Wn6_hyTkS&sssrc=4429486&ssuid=IE07kO4STZy&var=&widget_ver=artemis&media=COPY
How old is the battery?
That'd be my first port of call as a failing battery can cause various electrical gremlins or a diagnostic check to see if any fault codes flag up