I just did what you are trying to do over the Labor Day weekend. I put a trunk on my '05 150 and I added 2 double lights to it (these lights are typically used on the sides of trailers). I picked them up at a Harbor Freight store for $4.99 for a package of two. They function only as brake lights and not as running lights and they are not LEDs but with a total of 4 bulbs, they give pretty good visibility. Here is what you have to do to get to the wiring:
1.) Remove the trunk rack (take out the 3 hex bolts).
2.) Lift the seat and take out the 3 upper bolts and 2 lower nuts. Lift and remove the seat/storage compartment.
3.) Remove the black trim piece that holds the front of the 2 body panel halves together (be careful, these are both just plastic).
4.) Remove the screws that hold the left body panel and pull up on it firmly and carefully where it meets the floor board . You should be able to lift the panel enough now to gain access to the wiring without totally removing the panel (this is also how you gain access to the air filter!).
Note: I don't remember for sure how many screws needed to be removed for parts 3 and 4, but it should be obvious once you look at it.
5.) The center wire of the three wire harness is for the brake lights. I "tapped" into this and then used the screw near the gas cap for my ground.
6.) I drilled a small hole in the trunk and fed the wires between the storage compartment and the body panel to the wiring harness.
So far it has worked really well. Let me know if you have any questions. Good Luck!
