Somewhere in the dash wiring, you should find available outlets (on e-bodies, there are 2 sets of 3, they are small bullet-type terminal female receivers on orange wires. One is used for the dash light bar , which leaves 5 available (this may vary depending on the options). Don't use this for anything which would need any kind of heavy current (but a GPS and a phone is OK). I wouldn't draw more than two amps continuously from these wires.

I would install a fairly small (3 or 5 amp) fuse inline with the sockets just to make sure no-one plugs in a coffee heater or AC converter while I'm not there, and fry the wires.


Alternately, you could wire a relay to supply current from the power lines behind the amp-gauge or straight from the battery/starter relay (in that case, use a fuselink under the hood and a fuseholder and fuse inside the car), and switch it on using the ACC socket on the fusebox (if I'm correct, it's on whenever contact is on).