Should be fairly straight forward. (famous last words!)
Splice into the permanent 12v feed, route through a switch then splice the other end into the 12v ignition feed.
Not sure about the switched earth though (which may be used to power down after 20 minutes) so I'd wire the 12v+ 1st then test and if it turns off after 20 then you'll need to wire a permanent (preferably also switched) earth too.