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Should be part number 85626239 which replaces 84758465 and 85521472. Not sure if they "fixed" the switch or why the new part number. Mine quit working a couple of weeks ago after doing the spontaneous opening trick for a few days.

Gen5diy has a harness that I plan on using when I finally take the time to replace my bulbs. Kind of pricey, but they do appear to make quality harnesses. https://www.gen5diy.com/collections/19-20-silverado-products/products/19-20-silverado-drl-relocate-harness-for-halogen-headlight-models

For as often as I'm looking at the trucks data I really need to invest in something like the Edge Insight myself. Then I'd at least be able to save which data points I want to look at instead of scrolling through the laundry list of stuff my reader provides. I like to know what "normal" operation looks like so I can see when something goes wrong or isn't right.

Sadly there isn't anything the truck will display that will let you know when it's in regen, that's why I suggested the reader to be able to see that data.

When mine goes into regen the engine sound changes and my fuel economy tanks. I don't notice any weird blowing noise though, so it may or may not be related. If you have an OBDII reader handy, you may be able to pull live data and see soot levels and regen status when it's acting up vs. running normal.

Says it goes to "K9 Body Control Module". I'd think the BCM would need more than a 5A fuse, but I didn't look anymore into it.

First cargo lamp button press turns on all the lights, a second press of the button turns off the mirror lights, and a third turns them all off. Not sure if you can only have the bed lights on though.

I'm a fan of the electrical diagrams from the GM Upfitters website. You can get it at https://www.gmupfitter.com/light-duty-vehicles/light-duty-2020/. It's in the electrical section on page 218. Pretty sure there's some typo's on the diagram though. It has F4 through F8 in the wrong order. You can tell because F7 and F8 are definitely not 125A or 150A fuses. From what I can tell, your blown 100A fuse goes to "E40 Electrical Axuiliary Heater (C32)".

Yes, Google Maps has a Carplay app and can be used while driving. It works rather well and seems to be a lot more accurate than the Apple Maps app.

Might check to see if you have the plow prep package (VYU), as that removes the active aero shutters.

From what I understand the OEM LED headlight assemblies aren't meant to have the lights changed, as they are supposed to last 15,000+ hours. If something does go wrong, you'd have to replace the entire assembly though.

That definitely complicates the issue. It did get me curious so I randomly checked some of the various gm parts websites, like gmpartsdirect.com, and they all have that sensor in stock. If I was in your position I'd be seriously frustrated. GM owns ACDelco and can't provide the part while 3rd parties appear to have no issues keeping those same parts in stock. Plus they won't honor warranty work unless the part is sourced from GM.

If I'm not mistaken, I do believe that's the ambient air temperature sensor.

The HD AT4's aren't lifted like their half-ton counterparts, so the 2500 AT4 will be the same height as the Denali you test drove. The HD trucks stock height are something like 1" taller than the factory lifted half-ton AT4/Trail Boss. The ride would more than likely feel about the same between the Denali and the AT4. Something to look out for when test driving the HD trucks is to see if they have the snow plow/camper package that include the heavy duty front springs which would contribute to the harshness of the ride. I feel your pain in regards to not being able to find a 1500 with max trailering. I remember when I was looking for one the dealers typically had no idea what I was talking about or just say "oh yeah it has that" with no idea whether that was true or not. I ended up just looking in the glove box of the trucks to see if it had the NHT RPO code.

Just moving up to the 2500HD will be huge in making your towing more stable/enjoyable. So either the gas or diesel will check that box for you. Given the information you've provided the gas option meets or exceeds all of your requirements. For me it was easy enough to choose the diesel option, as it simplified my traveling while towing. I'm not saying the gas option wouldn't work for me, but what I was looking for was more in line with quality of life and future potential. Traveling long distances and through mountains is tiring and stressful when towing and now it's not much more worse than driving a car. The power going up the mountain and the the exhaust brake going down are the obvious benefits; depending on the traffic you can set your cruise control and it'll manager your speed going up and down. With my Pilot RV Plus card and my fleet card from TSD Logistics/Open Roads I can pull up to any truck stop and use the commercial lanes to fuel up and top off def at cash price or less. That simplifies trip planning and having to hunt for a big enough gas station to get fuel.