A multi-meter is key when installing a reverse camera and if it is not working, then the multi-meter will usually tell you where the break-down is and then you can solve from there. 

Many European branded vehicles will have a reverse bulb power that drops below the required +12v the camera requires to turn on / function. This is usually because the vehicle has a bulb monitor system (the dashboard will tell you when a bulb is out) or a networked lighting system via Canbus. The vehicle will use PWM to power the lamps. Pulse width modulation (PWM) is a modulation technique that generates variable-width pulses to represent the amplitude of an analogue input signal. So as the charging voltage the power is switched on and off several times a second. This means when you start the engine, you do not have a stable voltage feed that the camera system requires in order to function. Hence, it doesn't work when the engine is started on the vehicle. To solve the situation, we have developed our own reverse camera relay:https://audiotechdirect.com/atd-cca-18123-12v-dc-p... which will stabilise the voltage if taken from the bulb power source. 

Alternatively, if the head unit / monitor you are installing to is able to detect the reverse trigger via a decoder system, then you would not need to worry about using a trigger from the reverse bulb. You could take power directly from an ignition based +12v (ideally between 13-15v) to power the camera.