Hello peep's

my head unit is a Sony XAV-7W in dash LCD and the only way I could watch dvd's is by grounding a light green cable to the hand brake so when I have then hand brake on it lets me but i don't want to do that. I was thinking instead of doing that I would like to connect it to the fog light switch, so when im driving and someone wants to watch a movie i could just control the security of this LCD screen by just turning the switch to on.

Does anybody know how to do this? if so please help!