Power supply to towbar bypass unit

Hi all, I am fitting a new towbar to my vw golf mark 6 and using a bypass unit to fit the electrics.
The instructions say that you should use the battery as the supply to the unit but i have read thet you can take a connection from behind the 12volt plug in the boot as this will save any hassle running the power cable the full length of the car.
Does anyone know if the unit must have power to it at all times to ensure it works well or will it be ok to have the power going to the unit when the ignition is switched on and will this damage anything in the unit.
I am only towing a small trailer and only need it for lights ect and not for any other thing such as a caraven which may need a power supply when the ignition in the car is switched off.
Hope some one can help.
Many thanks John.:confused:

I'd also like to know as I am fitting one to my mk6. Have got towbar and electrics here ready to be fitted but just want to confirm this before I start.