> The IR transmitters sound interesting, and would lie somewhere
> between 1 and 2 above, but might be tricky to place so that they
> aren't also blocked by the passengers or cargo

If an IR transmitter pings into the car space with enough power
it may not matter that something is in the way. And it's unlikely
that something would be in the way as you get closer to the hinge.
People would generally be sitting way back by the door latch,
perhaps a good 60cm from the hinge

We know how sensitive a VCR receiver is if the transmitter is in
the room, but point the transmittion out the door and it's lost
-----------------
Is there enough of a round-trip resistance difference in a car
body when a door is open ?
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I wonder if ultra-sound might work. If you flooded the car space
with it, would opening a door cause enough of a 'leak' to detect,
over and above the movement of people within ?
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Detecting an interior ceiling light would be simple, but getting the
wiring tidy might not be easy for the mechanically-challenged
-----------------
Most cars now would have a door indicator light on the dash ?
Perhaps tap into its supply or a photo-detector on the dash (not
very aesthetic)

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