My dealer and pretty much every thread I have read online said it was not possible or financially impractical to add the factory controller to the 2014+ trucks if it didn't come with one. Which is why my dealer orders every one of their trucks with one.

But if you do figure it out make sure to do a detailed write up

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My 2016 Silverado ya RPO JL1 written in the glove compartment can I add the brake controller

 

The factory controller cannot be added. Period. End of story. If the dealer tells you it can, he'ss lying to you. Too much integration required along with programming, etc. RPO code doesn't matter. GM did not install "provisions" to make it available as an accessory kit.
Really, I fail to see the importance of it anyway. How much do you really mess with the controller anyway? My Tekonsha Prodigy can be set up for 4 different trailers, and the applied voltage can be independently adjusted for each trailer dependent on the load. What more do you need? It's not like you're constantly monitoring the applied voltage, or you're going to use the emergency lever....I would think transmission temp is something more important to monitor, or perhaps your speed in certain areas.
I said in a previous thread I installed mine out of sight, out of mind. PM your email and I'll send you pictures, but I think my install only works if it's the silverado with the jump seat.

 

It needs the JL1 RPO code

https://static.nhtsa.gov/odi/tsbs/2021/MC-10206716-9999.pdf

 

And quote from that tsb:

This is to advise you that, if the vehicle does not have the option (RPO JL1), it cannot be added to the vehicle.

 

For people wandering about the JL1. Most of that are claiming that your truck isn’t factory installed JL1 and that you do have the JL1 module beneath the bed above the spare tire. That module you are seeing is the fuel pump driver module which mounted to a bracket on underside of it. For JL1 there is a K38 chassis control module that is mounted to the top side of this bracket, a K133 trailer brake power control module that’s mounted beneath it next to the fuel pump driver module. The wiring for S76 trailer brake control switch is in fact pre wired through the dash and is terminated at the X138 connector. The wires needed to control the oth other to modules is not there. This very well can in fact be added after the fact providing you have factory wiring schematics and the knowledge of electrical connectors to research what connectors are used and what terminals are needed for each of the various connectors you’d be pinning the needed wiring into. There’s also some GMLAN wiring that comes from the BCM and feeds into ECM, the electronic steering gear, K17 electronic brake control module, and from the K17 the GMLAN terminates with a R6A resistor. Depending on your truck, this GMLAN leaves the K17 and feeds into the magnetic suspension module then passes through and feeds into the K38 before termination into the R6A.

 

Can you install the wiring, buy all the modules, install them on the truck and plug everything together exactly like it would have been, if the truck had the option installed at the factory: Yes.
Can you reprogram the trucks modules, so that after you spent all that time and money doing the above, it all works: No.