It's not hard at all. Forget the stensil you don't really need it. My buddy did this on his 01 Ram 1500 and it came with the locations already set and all he did was drill, and install lights and wires.
I'd think it'd be cheapest to buy some clearance lights from www.AWDIRECT.com for 6 bucks each and then do the wiring yourself. So 30 bucks, and you've got clearance lights or go to the dealer (or your work in your case haha) and order their entire kit that includes plug in harness for over 100 bucks.
You can use tape to make the line across the roof, mark the holes, and drill and I suggest driilling from the outside in. You gotta drop the front of the headliner of course but you should have no problem with that.
Brian's right - I did mine. I bought my lights and harness from the Ford house for - shoot I think about about $80.
It was a PITA just because drilling 20 some odd holes in the top of your rig REALLY sucks! Just kidding! The worst thing about the install is it better be time consuming because if it isn't you need to measure again ( ) and working around the headliner is a pain.
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awdirect.com is right, they are a towing supply house, so they have tons of marker lights.....led's too i think.....the install is easy.....but just like anything on your roof(visor, antennas, sunroof) you gotta make sure to seal it very well or you'll have a wet headliner and all kinds of other problems. I would use silicone gasket maker.....its good stuff......
I had no idea you could get the lights so cheap from Ford. I think I'll be doing this install pretty soon. Fortunately, I've had a bit of experience with this from installing/fixing clearance lights on Peterbilts. That's good advice Wes! I typically use about a 1/4 bead of silicone around all holes and have never had a problem.