Help designing a remote battery pack.

NWCCTV

I have a friend that has a bit of a dilemma that I am trying to help him with. Here is the situation: He has a covert wireless camera. The Camera and Transmitter use the same 12V DC battery pack. Neither his recording software nor his camera have motion sensing, so they are on constantly. What he would like to do is come up with a way to remotely send a command to the battery pack that would turn the camera and transmitter on and off. Any ideas, suggestions, etc. greatly appreciated.