if a thief wants your car, he will get it no matter what. all you can do is slow him down or hopefully make him move on to an easier target.
you can add things such as clubs, brake pedal locks, etc. for visuals. you can wire in hidden kill switches and things that must be triggered in order to start the car.
using all those options plus an alarm (which you don't want to do) would be your best bet. you can add all the security in the world, but nothing will be able to stop a thief with a truck and trailer from hauling your car away.
best of luck.
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2005 s2000