Undo the 2 hinge bolts at front of the fusebox, pull it down and you can access all the connectors and wiring on the back for a permanent connection.
Or for the easiest ignition switched power source look for the big multi-way connector for the centre console (behind the radio), easily pulled out into the driver's footwell and has the ignition switched +12v for the lighter on it.
The ground wire is black on both sides of the multi-way connector and the ignition switched +12v wire is orange on one side of the connection and red/yellow on the other side. The 2 wires are next to each other (horizontally) in the 3rd and 4th positions from one end of each connector.
So you're looking for a slightly thicker black and orange pair, or a slightly thicker black and red/yellow pair (depending on whether you're looking at the male or female connectors) in position 3 and 4 on the back of the connector.
Then just strip back the insulation a little, solder your wires on and tape it up or use heatshrink, or even one of those scotchblock things if you must.