In a MX5 / Miata NA / MK1 there is no buzzer that warns you that you leave your car with the lights on. This modification took me about 10 minutes and is very easy.

I bought the following buzzer on eBay:
Buzzer eBay

Next you connect the red wire to the red and black wire behind the dash light dimmer. Next you connect the black wire to the red and white cable in the loom that goes to your door. And that is it. Now you have a buzzer warning you that you left your lights on, when you open your door 🙂