OK maybe I'm getting a bit carried away but heres two more ideas that that are not intended to cause harm just a liitle fun and if your clever enough may just work.
As I'm in Vic I dont know what the carpark situation is but it sounds like it's bit remote - I'm hoping it's sandy or dirt? if so keep reading otherwise stop now.
Ridiculous as it may first appear my devious mind has been devising a man trap which will not hurt the victim (...much) but may contain him long enough for someone to come to the rescue (if not give the %$%#er a good scare). No this is not an easy one and would take one of you with an engineering mind a little bit of effort and experimentation.
It goes like this ...(making this up as I go)

OPTION A:

1. You get one of those old vacuum cleaners and you put your ...hang on which forum am I on? ... ahh, thats right, you get one of those old vacuum cleaners with the retractable cord. The thing you want is the retractable cord unit - make sure that it has fast retraction.

2. Replace the power cord in this unit with some strong nylon cord or light steel cable.(kites lines may be an option but cause damage and be easy to cut).

3. Make a large slip knot on the end of this cord and have a 'lock' on it (like a cable tie) so it can get smaller but not open up again.
Also place a similar lock on the box housing the retractor unit so that the cable cannot be spooled back out without poking a screwdriver through a hole in the box to deactivate it (this is so your thief cannot run away by pulling the cable out again

4. Place an electrical actuator (like in a central locking system) on the retraction switch.
Connect the actuator via a wire to your car's interior light circuit (activated when any of your car's doors are opened). Connect your horn as well (dont use the interior light when mantrap not in use!)

5. Place a switch to deactivate you interior light in a discrete spot on the outside (or underside) of your car so you can active/deactivate the system from outside.

6. Place the unit under your car, or in the car or nearby bushes. Secure it to your car (exhaust or the like) or a tree via a cable and padlock.

7. Spool out the slip knot and lay it in a large circle (about 6-8 feet wide) on the ground below your car's doors. Placing the units inside your car is riskier then the cable will run out the door you expect to be opened and will also require some disguising where the door closes on it.

7. cover the slip knot with sand or dirt.
8. Carlessly leave some goodies in your car.
9. Stick a video camera in the bushes because I want to see the action!
10. Go kiting!


OPTION B:

1. Precariously hang a full size billard table in the tree above your car (maybe borrow your friend's car for this one) with goodies inside.

2. Connect a rope from your car's doors to the billard table.

3. Go kiting. (Do not video this one.)




....Well what can I say - it's quite at work tonight. Good luck and remember to video option A.