In message , Adrian Brentnall
writes

Just a thought....
If you have a voltmeter of some sort - connect it across the 12v mes
bulb in the above setup.
Naturally, there'll be a voltage drop across the bulb when the door is
opened, but is there any volt-drop across the bulb when the door is
closed..? If there is, that indicates that there's a small current
'leaking' through the interior light, via the door-switch.

OK, good idea. My multimeter is an ancient Eagle analogue, which is
great for current is/is not there, but difficult to detect minor (ha!)
variations, particularly as the closest scale is 0-50v. I'll have a go,
though.

Car batteries are designed to provide lots of current for a short
period of time, to start the engine. They're not good at long-term
supply of smallish currents... - and the car ammeter won't indicate
accurately down at that level...

I know you're right. I'm sure I have an analogue 5amp meter somewhere,
but even that will probably not detect a small drain. Will try anyway.

Come to that - can you put a test meter across the in-car ammeter?
With everything off there should be no voltage. A sensitive voltmeter
will tell you if there's a small current being drawn from the battery,
even when everything on the car is 'officially' off..

Say that again. Assume I have the ammeter (which is not fitted in the
car) between the battery -ve terminal, and the earth lead. Where do I
connect the volt meter?
--
Graeme