Rear parking Sensors

[Kamerman]

Hi guys,
I bought aftermarket rear parking sensors, and I asked an electrician to install them.
After the job was done the sensors didn't work properly, the beep as not constant (it beeps then stops then beeps again).
I changed the module and the buzzer but the problem maintains.
I asked him where did he connected the live wire, and he said on the reverse wire.

[Kamerman]

Update after so long.

The problem was in the parking sensor module.
Has to try it 3 times, they said it was a product defect.