I just bought a uses 2017 Sierra 2500 Denali and decided to add the OEM roof Marker lights. After EXTENSIVE research online (this forum and several others - thanks to all those that have posted installation guides!), I thought I'd add some helpful installation advice I didn't come across in my research. So, this post is not a step by step guide because there's at least a half dozen of those out there.
1. Measuring and Template Setup - yes, setting up a cardboard template cut to size is a must. Initially I had a very hard time because I was trying to using a tape measure, square, or metal yard stick. These are a waste of time. All the changes and curves to the profile of the rood and windshield make it very difficult to get precise measurements using something rigid, especially by yourself. After 2 hours I still wasn't happy with the light placement, I went through my wife's sewing kit and got the flexible vinyl measuring tape used by tailors...so much easier.
2. Cutting the holes - I found this kinds funny. All the other posts talk about how you should use the Greenlee square punch, or how you must absolutely avoid cutting it too big, so it's better to cut smaller and grind it larger if needed. First, the cheap maroon Dremel cutting disc's work amazingly well. On my first square, I intentionally cut on the inside of the line...don't do this. You'll spend another 20 minutes grinding and filing all the sides to get it to fit, and the amount of metal shavings increases 10x. After inspecting how the lights are attached with 4 prongs that slide outward onto the metal roof as the bolt is tightened, I could tell that EXACT precision was unnecessary. The remaining 3 cuts took about 10 minutes. JUST CUT IT, and I actually starting aiming outside the line so I wouldn't have to do it again. The Dremel disc's worked out perfectly because they start at 1" diameter, and after cutting all 4 sides of the square, I discovered the disc's were reduced to about 3/4". So to smooth out the corners and sides, I just turned the disc vertically and went from front to back on both sides of the hole, making a perfect square. This works especially good on squaring off the corners. I used one disc per square. Back to the level of precision required...without a doubt, you can be AT LEAST 1/16" off on EACH side and have no problems securing it, maybe more. I had one hole like this and it actually worked out because I was able to reposition it slightly before tightening. But you definitely want to put a bead of silicone around the edge. It's also a good idea to put a sheet in the interior to catch the shavings. Some people mentioned that their lights weren't flush with the roof on all sides. Mine were like this too until I tightened the SH!T out of it. I don't know what torque I was using, but I'm 230 and bench 350 lbs, and I was probably giving it about 75% of what I could do with a small ratcheting wrench. Just keep tightening slowly until the light doesn't move at AL when you push and pull on it, or when it's completely flush.
3. YOU DON'T NEED NO STINKING HARNESS - this is probably the most useful info I'm sharing. I actually planned on buying a harness, but after checking with all the dealers around me (for each of the 5 or 6 different harness part #s floating around in the forums), nobody had any in stock. My wife and kids were out of town, so I knew I had the day to myself...wanted to install it that Friday.
I tested pin 3 in the passenger fuse box, and it provided the correct signal. I wanted to a avoid cutting the plugs on the lights, so I was digging around through the leftover/junk parts I have after 20 yrs of 12V installs, and came across an old wiring harness for an aftermarket radio, and the flat pins looked like they'd fit into the light plugs...and they do! So I pulled the wires and attached the to the light harnesses and crimped heat shrink bullets on the other end.
Then I added 2 pcs of heat shrink (adhesive lined), 1 on the exposed metal of the positive wire (to prevent a short), and a larger one over both wires and the plug to hold it in place.
Next I went looking for a female connector that would fit into slot 3 on the factory harness. I found a donor on an old HID bulb harnees. I cut it out of the plastic and after a little reshaping with some pliers, I was able to add it to the factory harness. I've hit my limit on pics, so I'll try to reply to this thread with some later.
So, I got power from pin 3 and ground from a chassis bolt by the fuse panel, and just ran 2 wires up the pillar next to the original factory harness. I connected everything under the headliner using bullet connectors. I added 3 females bullets to the power and ground wire, which is then connecting to the 3 sets of male bullets from the lights. I was able to squeeze three 18 gauge wires into one blue 16-14 gauge female bullet.
Crimping and heat shrinking all these connectors, and very meticuloslusy chaining them together took a while. I wanted to be able to unplug everything easily (like a factory harness). If I were to do it again, I would just use crimp caps... that would have saved at least an hour.
Then I zip tied the wires to the factory roof harness and made sure everything was tucked away and secure. It works and looks AWESOME.
So...you don't need no stinking harness!
I was able to wire this up 100% FREE by using spare and leftover parts and wires in my basement. AND, I didnt have to remove the dash to replace the factory harness. As I mentioned, I can't add any more pics, so I'll try to reply with some later (this is my first post).