You are absolutely right of course and I should have disconnected the battery. I assumed that I wouldn't need to disturb any connections whilst running RCA cables - unfortunately I succumbed to a 'I wonder what happens when I wiggle this obviously-corroded earth connection?' moment (I'm prone to those
). Thank you for all the suggestions so far - the 'open door' display isn't working and so my next stage is to check the wiring running up to the BCU and then probably buy another BCU (I'll worry about key fobs later)
Andy