I purchased this LED third brake light here:
http://www.streetbeatcustoms.com/wantb203.html

and it says that it is plug and play with no splicing. I have a 2004 xlt supercrew, I popped off the old brake light and found no sort of plug and play device. The only thing present were three different wires going to each of the three lights. Any suggestions on how I go about installing this?

 

I had the same problem.

You need to remove the two fasteners for the headliner that are towards you rear window, just pull on them and they will come out. Then pull the headliner down, it won't come down much, and then you should see the plug, then simply undo it and then connect the aftermarket one back in.

 

Toward the passenger side there is a harness mount. It is fastened to some sheet metal with a trim 'christmas tree' stud.

Pop that loose and behind it you can get to a harness plug. Then you can detach the stock 3rd BL and plug in the new 3rd BL.

 

Thanks a lot. I got it installed but the only thing I dislike is that face that the white leds dont light up when I put the truck in reverse.

 

The stock ones don't light up when put in reverse either.

 

I was always under the impression that they did. It makes sense for them to do that, but I guess they dont lol.