Quickly Googled this and found three ready made solutions.
I'm liking Solution 1 - what do you guys think?? Buzzer will only sound when ignition is OFF, BUT is it a good idea to have the relay permanently live while the ignition is on? Will it not burn out???
Solution 1
When the ignition is on the relay holds two contacts apart. When the
ignition is switched off the contacts close and connect the buzzer to the light circuit. If the lights are on then the buzzer sounds.
http://www.gbnetwork.co.uk/circuits/carlights/car_light_reminder.html
Solutions 2&3
These two headlight reminder circuits are easy to install and operate on the KISS (Keep It Simple Stupid) principle. The simple circuit involves adding just a 12V piezo buzzer between the lights circuit and a door switch. The buzzer sounds if the lights are left on and you open a door. The disadvantage of this simple circuit is that it's annoying to have the buzzer sound continuously if you want to leave the door open while the lights are on. The improved circuit
overcomes that problem by adding a 1000μF capacitor and a parallel 100kO resistor in series with the buzzer. Now, when a door is opened, the buzzer gives a brief burst of sound only, while the 1000μF capacitor charges. The 100kO resistor discharges the capacitor when the lights are switched off.
http://www.circuit-finder.com/categories/automotive/automotive-light/693/simple-headlight-reminders
I'm liking Solution 1 - what do you guys think?? Buzzer will only sound when ignition is OFF, BUT is it a good idea to have the relay permanently live while the ignition is on? Will it not burn out???
Solution 1
When the ignition is on the relay holds two contacts apart. When the
ignition is switched off the contacts close and connect the buzzer to the light circuit. If the lights are on then the buzzer sounds.
http://www.gbnetwork.co.uk/circuits/carlights/car_light_reminder.html
Solutions 2&3
These two headlight reminder circuits are easy to install and operate on the KISS (Keep It Simple Stupid) principle. The simple circuit involves adding just a 12V piezo buzzer between the lights circuit and a door switch. The buzzer sounds if the lights are left on and you open a door. The disadvantage of this simple circuit is that it's annoying to have the buzzer sound continuously if you want to leave the door open while the lights are on. The improved circuit
overcomes that problem by adding a 1000μF capacitor and a parallel 100kO resistor in series with the buzzer. Now, when a door is opened, the buzzer gives a brief burst of sound only, while the 1000μF capacitor charges. The 100kO resistor discharges the capacitor when the lights are switched off.
http://www.circuit-finder.com/categories/automotive/automotive-light/693/simple-headlight-reminders