This is a very simple circuit to make using a 555 component and is recommended for the electronics beginner.

This page outlines how to make a simple theft detterant which may be just as effective. The idea is to have a flashing red led indicate that your car is protected. This device can protect your vehicle from potential thieves - it makes it look like your car has an armed alarm system installed, which causes thieves to pass by your car in search of an easier target.