After installing my keyless entry, I often found that I would like to be assured that the car has locked even when I am too far away to hear the lock actuators go "clunk." Most alarms that have remote locking flash the lights, but my system is not that fancy! I ended up installing a large, loud (so loud that I have had to attenuate it with a strip of tape over the opening) piezoelectric "buzzer" outside my rear door, but behind the plastic cladding. I originally mounted it inside the car (much easier!) but it was too loud if the car were ever locked with occupants inside. The buzzer is actuated by the same 12V signal that powers the lock actuators. That signal is 0.2s in duration, and is just the right amount time to make a loud chirp without being too annoying.
One of the difficult parts of the project was removing the rear cladding. Before doing anything else, remove the nut that is shown here by the arrow. All other mounting points are clips, and after much trial and error, I found it best to leave the clips in the metal door, and remove the plastic cladding from the other end of each clip. It isn't easy or fun... but it IS possible. Also in this picture you can see the two black wire taps just to the right of the mounting nut. The tap with red paint around it is the positive lead (goes to blue wire). The other is the ground (goes to red wire - go figure!). My first attempt had the thing chirping during UNlock. So much for reading the wiring manual correctly! This view is possible only after removing the water-proofing plastic from around the inside of the door that is behind the pretty plastic interior cladding.
Here is the dirty, dusty area between the cladding and the back of the rear door. I stuck the buzzer to the box that is formed for the license plate, and put a shroud over it to keep the water out of it - then gooped the top with silicone. I have no idea how long it will last here, but I've given it as much chance as I could! This orientation gives plenty of airspace to fire into. It comes out as an omni-directional sound that isn't easy to locate. In this picture you can also see the wires passing through the existing grommet in the middle of the door. Power for the license plate lights (now LED too!) comes through the same way. After some hit and miss, I have attenuated the volume (with tape) so that I can easily hear it out to the range of my remote lock signal - while still having the sound low enough not to wake the living dead if I lock the car late at night near somebody's house.