The switch can be almost anywhere. What I'd do, is run an ign power lead (one that is hot when the key is on) to the switch, and then on to the relay. Then, you leave the switch 'on' except when you want to play the stereo for long periods of time. The relay will open every time you shut the car off, which will keep the batteries from running each other down. Just make sure you flip the switch 'off' when you are playing parked, or all your work will be for nothing.

Toby