Just one more thing, you need a double pole switch, since when it is on the vehicle battery it needs a connection to two separate power feeds (the one that maintains the stereo when the engine is off, and the one that wakes it up when you start the engine). On hab power, both of these come from the leisure battery 12V.
Here's a photo I took of the switch as I had wired it up ready to go in.
Pink wire to hab 12V (leisure battery). Red and yellow wires from the middle two pins to the radio wiring loom to match the red/yellow wires there. The other red/yellow wires from the left hand end to the van wiring (again should be red/yellow there). I think that the yellow is 12V permanent battery and red is the one that comes on with the ignition. Note that the pink wire connects to both connectors on the right hand side.