[Eman]

You can run it back directly to the battery...but it defeats the purpose of having a cutoff switch at the back of the car. The cutoff will shut the car off but it obviously won't kill all power since that alt wire will still be hot. And for the alt to charge properly you really should use a heavy cable like a #4 all the way back.
There's a few ways to accomplish shutting off all of the power. One is with a continuous duty solenoid or relay. It will only activate when the batt switch is on and shut power to the alt. when the batt switch is off. you can also buy a double pole single throw cutoff switch like a Moroso 74102 or a Cole hersee 75912. it has an extra set of contacts that shut the alt power off