Just a thought.

Thanks, I'll see if I can make sense of the wiring diagram.

....the Jimny auto gearbox wiring diagram.

For anyone else trying to do this I've made a post building on what was in this thread:  bigjimny.com/index.php/forum/6-jimny-chat/57617-head-light-warning-buzzer#240406

The one I have set up is triggered when the headlights are on but the ignition is switched off .

More out of curiosity than anything else, doesn't the Jimny come from the factory with a "lights on" reminder?

The last vehicle I had that didn't come with one was a 1983 850 Mini - the one on my 92 Swift would sound a continuous tone if the driver's door was opened with the lights on and the engine off (forgot lights alert), and a repetitive beeping if the driver's door was opened with the engine off and the key in the ignition (forgot key alert), and there is similar functionality on the three Suzukis I currently own.

Aren't we "reinventing the wheel"?

The wiring diagram lists it is a having one but some very early ones (like my 1999 model) do not seem to have one - maybe it was broken?

I suspect that the one on your 99 is "broken", those "buzzers" have been standard fittings on Suzuki since the late 80's early 90's.