First of all, you do not want to change the lighter socket to switched power; it is configured to always hot so you can plug a battery maintainer in without having to leaving the car unlocked or exposed in any way.
Under the radio and behind the console is a plug meant to be for the factory cell phone. This plug has switched, unswitched, and chassis ground connections that is perfect for your project:
It is easily accessed by simply pulling off the console kick panel on the passenger's side.