As Whyman says, forget about the switched 12v, it needs constant power and it won't drain your battery.
I fitted a tracker to mine a while ago and I put it behind the plastic cover under the boot floor (Shhhh... don't tell anyone
). I took the power from the cables that connect to the trailer module, one of which is 12v constant live.
I've been thinking about fitting a second tracker (because now they know where the first one is) and I'd thought about whipping the glovebox out and tapping into the 12v feed from the battery and hiding it somewhere up behind the dash.
Hope that helps.