Reverse Camera Display Issue

My reversing camera installation in my Van has a weird issue that has taken a while to track down.
I use the 12v supply in the van to power the camera (not the supply from the car). When I use the display in the van all works well.
When I use the display in the car all works well until I hook up the van running lights to the car.
Then all I get is a blank display.
It has taken some time to get the exact situation so I know when it works and doesn't as I usually hook up the running lights first.
The two systems are independant other than they share the same earth (ie: the van chassis). All other power systems work fine and the display seems fine otherwise.
I know the real simple answer is to use the power that the running lights are using, but I had prewired everything as I was building it and to re-wire it now is a major effort.

Any ideas (other than saying simply it must be the earth).

Cheers Cam