Few places you can get the interior light circuit at, depending on where you are planning on running the wire out of the cab of the truck.
You are looking for a black w/ light blue wire, which is the non dimmed interior lamp circuit. This is the same circuit that is used for puddle lamps on the mirrors ( if equiped ) and the cargo lamps in the CHMSL.
The common splice seems to be in the driver's side C-pillar, this is for the split between the CHMSL cargo lamps, and the puddle lamps.
What might be a bit easier is in C238, in the driver's side kick panel. This is a 60 pin connector, mounted to the metal on the A-pillar. It is pin #6, just to the side of the center bolt holding the connector together.
The other is behind the dash, up high ( in the area under the cluster "hump" ), to the right hand side in a cam over connector ( attached to the cluster itself ).
splice and tap are used interchangeably, but to be 100% correct, a tap is a splice that the main wire is not cut. This might also be called a T-tap. There are crimp type T-taps, and the solder method of T-tapping.
This is direct from the
technical article here at F150online.com
The URL is in the header in the black band.
Look at the top of the web page, it is right below the word unofficial in the gray banner. Or here :
http://www.f150online.com/tech/index.html