I recently traded in my 2017 Sierra Denali for a 2020 Sierra Elevation that didnt have the factory brake controller. After some research, and my dealer's recommendation, I ended up putting in a Curt Spectrum controller, C51170.
The brain mounts hidden underneath and it has a knob that can be mounted wherever you want. I chose a spot to the lower left of the steering wheel so only I can see it but still have easy access. It's as close to an OEM look as you're going to get while still having an excellent brake controller.
It is also super easy to install as its completely plug-n-play with the purchase of the OEM connector, C51525. It took me longer to tie wrap the wiring to make everything look nice and clean than it did to install the brain and the knob.
etrailer.com sells the package together in the link below.
https://www.etrailer.com/multi-product.aspx?pc1=C51170&pc2=C51525&VehicleID=202090028&HHYear=2020&HHMake=GMC&HHModel=Sierra+1500
Hope this helps!
Jeremy
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