I thought it might be useful to connect a positive throw for the reverse camera through a diode. So I've currently picked up the reverse light positive cable, the one that 'flickers' around 10V, and connected that to a miniature relay which starts to switch at 5V. The relay simultaneously powers the camera and also grounds pin 17 on the TV module.
This circuit works well enough with my NTSC camera on my UK E53. In the future I might look at other sources of video which can be added and switched in, this circuit lets me do that.
The relay switches with about 30mA draw, the camera is specified as 300mA, I'm not sure what Pin 17 needs but this circuit delivers it. No problem with the check control on the reverse light, either.