The relay should only click ON when reverse is selected.
Pins 85 & 86 are the relay coil.
Pin 85 should be to the reverse lights, pin 86 to ground (or vice versa, doesn't matter)

Then the relay contacts switch the rev camera control line
87a and 30 are connected together when the relay is OFF
87 and 30 are connected together when the relay is ON
So connect pin 30 to ground
And connect pin 87 to the rev cam input on the video module

Voila, control of camera input will now work