reversing camera

Hi, i have a superb estate and i want to fit a reversing camera. I tow a trailer quite often and i want a camera to help see the tow ball when reversing into the trailer. I bought a wireless camera system on ebay which consists of a 170deg camera and a wireless transmitter. I plan to power these two items from the reversing lights. Can i feed this directly from the reversing light wire?? The power required is about 3 or 4 watts. Will this affect the canbus system?? I plan to fit the camera between the boot opening button and the left number plate light. Bring the lead into the boot door through the number plate light and then place the transmitter inside the boot door. Take the power lead through the rubber boot and down to the reversing light.

At the front end i want to fit a small 3.5inch screen and a wireless receiver. I have wired both the camera and the screen and powered them using the two cigarette sockets and the system works well. I want the screen and wireless receiver to be powered from somewhere which switches off with the ignition. The screen when powered remains blank until the reversing camera is powered up (by the reversing light). So if the screen is only powered when the ignition is switched on there will be no battery drain.

The cigarette lighter seems to be powered constantly, can anyone tell me where i can join into a powere source which is switched off with the ignition please.

Thanks in advance.