Run a power wire up to the power module (fuse box) in the engine
compartment, and tap into fuse #33.  This is the circuit that powers the
trailer running lights.  Alternatively, you can tap into the 18-gauge brown
wire running along the wire bundle down the driver's side frame rail.  Test
this with a voltmeter, to make sure it's hot when the parking lamps are on,
and that it goes dead when fuse #33 is pulled.  If so, that's your wire.