I think I have found the solution here:
The reverse switch on the back of the head units require a +12v connection into them, If there is not +12v then it won't switch. Basically on the canbus European cars, because they have a very integrate canbus system and the vehicles will often tell you when bulbs are out on the vehicle, the voltage on the reverse bulb will drop from +12v (on key turned to accessories) to below +12v, even sometimes as low as +9v, when the engine is started. Hence this is why the trigger for the reverse isn't working when the engine is running but when you turn the engine off and put the keys into accessories position, it may display when in reverse. The canbus system controls the voltage and ampage of the reverse bulb positive pole and because of the actions that happen when the engine is started, this is why it fails.
A relay is going to convert the reverse bulb voltage into a stable +12v voltage for our reverse switch on the head unit. We are going to need to wire the reverse camera like this:
1. Rear Camera DC plug (red and black) - The red wire needs to be connected to a +12v switch ignition source. The black to an earth point.
The rest, we will be using the relay.
2. Pin 87 - The red trigger wire on the RCA plug needs to be plugged into here and only here (the other end will be connected to the head units "reverse" or "back detection" wire)
3. Pin 86 - This is where your reverse bulb positive pole wire goes into (you would spur a wire off the reverse bulb positive pole and into this relay pin position)
4. Pin 30 - Spur a wire off the red DC power plug into here. This will now be a +12v switched feed going into the relay.
5. Pin 85 - Earthed to chassis.