First, I'd like to give thanks and credit to the forum for an earlier thread I searched up. I needed help with doing the step install. That thread showed me it could be done, but didn't explain how. The '11 and up trucks of course have the urea tank saddling the frame on the passenger side. GM designed 2 sets of special brackets to install near the urea tank. These brackets are designed to hold the weight of the loaded step, but are different because of the urea tank. The tight clearances in that area make installing the upper forward bracket a major challenge. Some have advised blowing this off, but my eval tells me the pinch edge of the rocker panel will deform over time without the upper bracket. I got it done.
Remove the lower protective cover and insulation from the urea tank. 3 bolts and 2 hangers hold it up. 5 minutes or less to remove. Set it aside.
I recommend you install the 2nd pair of brackets at the rear of the urea tank first. Relatively easy, and you'll know precisely how the front brackets must go when these are done. Trust me.
First, find a mechanically-inclined guy with a slim arm, Fatties are out. Remove any splash guard and fasteners along the outer edge of the front passenger wheelhouse liner. Turn the tires all the way LEFT after doing this. The guy with the slim arm is up to the plate now. Pop out the black plastic cap which will allow the u-nut to be clipped in place. Using a long screwdriver and good 3M double-side tape, fasten the u-nut to the screwdriver tip. Reach in and position the u-nut over the edge of the hole, and use a small curved bar (narrow pry bar) of some sort to force the u-nut into place. A straight screwdriver won't work. Rotate the u-nut until the threads are properly positioned.
Slide the upper bracket in from the front, between the urea tank and rocker panel. Get it positioned so the upper slot is lined up over the threads in the u-nut. The most difficult part is hand-threading the bolt thru the bracket and into the u-nut. No tool will work that I can think of. Skinny arms and nimble fingers will eventually get that bolt started and turned in finger-snug. Before tightening it, install the 2 lower, smaller bolts which hold the upper and lower brackets together. Tighten these to final spec, and then do the single upper bracket bolt. A box-open wrench will finish it, but a 13mm ratcheting box would be easier.
Reinstall the urea tank cover and insulation. Replace the wheelhouse liner fastener and splash guard if you have one.
After these 2 bracket assemblies are installed, put on the plastic bracket covers. No steps have yet been installed, but 85% of the work is done. Just follow the mfr instruction sheet to finish them up. Do not lose any u-nuts or bolts, as no extras are included.
After fighting with mine and facing defeat, I employed a second guy. He did the "skinny" work. I gave him precise verbals and fed him nuts, bolts and tools as needed. From dead scratch, I think you need to allow 2 to 3 hours for the work. Not fun. Doable...but be ready for a challenge! My steps are all on, and they look great. I enjoy the way they hide the urea tank and frame rails from view. Maybe I'll take a picture tomorrow.
Not sure how different the 12 is from 11 or if I even have the extra insulation, but I used a few extensions and socket to go over the downpipe, over the frame and over the def tank to get to the top bolt on the front passenger bracket.