BD139 (T4) is an npn medium power transistor. A 5.6V zener diode (ZD1) is connected to its base. ZD1 provides 5.6V DC regulated power supply to the horn circuit built around transistors T1 through T3. Diode D1 functions as reverse polarity protector. 12V DC to the circuit is connected via brake switch of the car/ three-wheeler.
When you reverse your vehicle, the circuit is connected to the battery of the car through the brake switch. The AMV oscillates and produces an audio frequency, which is given to speaker driver transistor T1 to generate a beeping tone. This tone can be altered by changing the values of capacitors C2 through C4 in multivibrator circuit.
Assemble the circuit on a general-purpose PCB and enclose in a suitable box. Connect 12V DC power supply to the circuit through the brake switch. When you reverse your car the circuit gets 12V required for its working. Place the cabinet in the vehicle such that it produces sound at the rear while reversing the car.