Find a different dealer, it's easy to do, the plug is already in the dash behind the cubby where the controller goes. If you are handy, you can download forscan and apply for a free license and enable it in the software for free. Any decent dealer should know this and probably charge you 100 bucks labor to enable the feature. Being an EL, you already have the 3.73 gears and aux transmission cooler (you might double check this, it's a 7 row cooler in front of the radiator under the hood). All you need is the brake controller and the 7 pin connector wired to the hitch
I have an EL but there is no Aux transcooler that I can see. It looks like the plastic mounting bracket it there but no cooler.
I will at least be adding the HD radiator. I’ll price out all the hoses along with the cooler and decide if I add the cooler or not.
But back to the OP.
If you want the brake controller just buy it for like $75. Remove the left cubby, fish out the wiring harness, plug it into the controller, pop the controller in the place of cubby.
Download ForScan, make the needed programming changes.
Buy a 4pin to 7pin adapter and add the capped wired in the rear and you are golden.
You’ll be about $125 all said and done presuming you need to buy an ODBII module to do the ForScan programming.