No. After 2007 it was an option though.
BUT, it is fully prewired for one all the way back to the 7way connector. (if you have the towing package, if not it should still be wired all the way back to the junction box under the bed that the harness would plug into)
Wiring one up is really easy as well, just four wires, and if your Denali came with the towing package, it would have come with a 4 wire harness with a fuse taped to it in the glove box. The fuse goes in the fuse box, replacing a red block off, this provides power to the 7way connector to charge a auxiliary battery for a camper or 12v winch on car hauler, etc. The harness plugs into a junction box above and to the left of the brake pedal, wire the other end to the brake controller and you are done. It takes 5 minutes to install a controller on one of these trucks.
That harness may have been lost to the previous owner, if so you can pick it up at any local truck/trailer shop that sells brake controllers. I actually have a spare one lying around. Beware, 99-02 and 03-06 use the exact same connector BUT they moved around a couple pins in the connector, you can fix this by swapping the pins, or just swapping the wires when you hook it up (or just buying the correct harness haha) I found this out the hard way when swapping my controller from my 01 to my 03. Turning on the headlights resulted in FULL braking being applied to the controller, glad I found this out before hooking to a trailer, that would have been interesting about the time the sun went down on that first tow
I already had it wired up, so I just pushed the pins out and put them in the correct spots after finding a diagram online.
