When someone opens the car door, switch S1 becomes 'off' as shown in Fig. 2. As a result, transistor T1 conducts to fire relay-driver SCR1 (BT169) after a short delay introduced by capacitor C1. Electromagnetic relay RL1 energises and its N/O contact connects the power supply to piezobuzzer PZ1, which starts sounding to indicate that someone is trying to steal your car. To reset the circuit, turn off switch S2 using car key. This will cut-off the power supply to the circuit and stop the buzzer sound.
Fig. 2: Wiring diagram for door switch