So I had a go at this last weekend and all seems to be working correctly.
What I wanted was an indication that the lights were left on with the key removed when I opened the drivers door. Connections I needed were a negative from the opened door, a positive input from the lights and a positive from the ignition with the key inserted.
Got relay from Halfords and buzzer from Maplins.
Door negative: I couldn’t find the negative wire from the door light switch behind the dash so ended up having to go to the switch itself Had to drill a hole to feed my connected wire up behind the binnacle.
Lights-On positive: Got this from the little light that illuminates the cigarette lighter.
Key in ignition positive: Again got this from the ‘actual’ cigarette lighter as it only works with the ignition on.
Exploratory surgery.....
Wired it all together as diagram.....
I just fixed the buzzer behind my centre console with Duct Tape, and as it was a bit loud, at 80dB, I covered it with a strip of tape as well!
All working well.....so far