When I look at the picture, the way I understand is like this.
The relay will connect the 2 wires from the VGA box when you put in REVERSE or/and flip a switch (blue 12V constant box).
If that the case, you should put another diode on the switch side. You don't want the power from the Reverse side interfere with the switch's source right (voltage different btw the 2)?
If you want the VGA box switch only when you shift to REVERSE, take the switch part out.