Adding late to this topic, I have added a Rover YWC104500 multi-function unit (R25/45) to my MPI which are available for ~£10-15. Picking up ground, constant power and switched/acc power from the radio's feeds, 'lights-on' from the clock illumination feed and door open from the LH door switch line (the MPIs have a single line for both doors).
It works well: tone sounds when lights on, ignition off and a door open. The unit is a bit tricky to mount as does not have mounting lugs but managed to strap it down below the glove box.
Edit - just to add worth getting a unit which has been cut out with the connectors as these are difficult to source (though I understand RS stock them).
Edited by Deeppockets, 14 August 2021 - 04:24 PM.