The usual DIY job is to connect a cheap piezoelectric buzzer to the ashtray light (+12 volts if the lights are on) and the middle solder tag (IIRC) on the back of an interior light (Ground, if the door's opened).
I found I needed to put a diode in series with the buzzer, otherwise it squeaked feebly all the time.
An added subtlety is to put a relay in series too, which disconnects the buzzer if the ignition's on. Stops the annoyance of the buzzer going constantly when, for example, a passenger's getting in or out while the car's running & the lights are on.
Diode & relay ratings - they're only carrying the current for a little buzzer, so just about anything will do.
For the diode, something like a bog-standard 1N4001 would do fine. (Or 1N4002, 1N4003 etc.) The relay should be a 12 volt one, and needs to be a changeover type because you'll want to use its Normally Closed contact ("87a" contact, in an automotive relay) so that the buzzer can only operate if the relay is not energised - i.e. when the ignition's off.
So then you need to connect the 3 components in series. Find the contact on the back of one of the interior lights which switches to ground when you open the door. Wire from there to the cathode of the diode (marked with a stripe) and from the other side of the diode to the "-" terminal of the buzzer. Then from the "+" side of the buzzer to the "87a" contact of the relay, and from the "30" contact of the relay to the side of the ashtray light that goes to 12 volts when the lights are on.
When you've done that, the buzzer should work when you open the door with the lights on.
To kill the buzzer when the ignition's on, you need to find a source of 12 volts when the ignition's on and a source of ground (I think I found these on the back of the heated rear window connector. It'll be behind the dash, even if you don't have the switch installed). If you connect those two points to the relay coils ("85" and "86" on automotive relays) then the relay will energise when the ignition's on, disconnecting the 87a contact and disabling the buzzer. Job done.
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