Hi Omne,
I had an old motor with a battery that was not too clever.
If i left the side lights on when i left the car, it would
not start unless i was very quick to get back.
After this had happened a few times, i decided that i had
to fix up a buzzer or something to make sure i switched
them off.
My first arrangement was very simple and very, very
effective.
It comprised of a horn relay and a silicon rectifier
between two parts of the car wiring.
It made the hooter sound if i opened the car door with the
lights on.
I very quickly learned to switch off the lights before
opening the car door.
The hooter would stop of course when i closed the door,
so if i wanted the side lights left on, that was ok.
The interior light was operated by the door switch
applying a chassis (body) connection to the interior
light circuit to make it light.
The external lights on the car all have one side connected
to the chassis (body) of the car, and require a feed from
the battery to be applied to make them light up.
On cars here, the usual arrangement is that the side lights
come on when the switch is actuated, and when the switch is
actuated further the main lights come on in addition to the
side lights. So the side lights stay on when the main lights
are operated.
The hooter is operated by a relay.
The horn relay is operated from the steering wheel by
applying a chassis (body) connection to the relay to make
it operate.
What i did was to connect a silicon rectifier diode and an
old horn relay coil in series between the interior light and
the external lights, mounted on the bulkhead under the bonnet.
This did not interfere with the hooter.
And it did not interfere with the interior light.
It was effective, although i changed it later to work
a buzzer which i salvaged from a different car.
The diode is there to reject current from between the lights.
The changes i made, i have drawn in blue.
Almost a year later i changed it to work a buzzer instead.
Best of luck with it,
John