If you bought one and all the paperwork is done, and NOW you want something different or extra, I'm afraid you have made a mistake and your wallet is going to pay dearly for it.
There is a whole rigamarole with the rear bumper trim/fascia being different for those with the tow package. There are some details around the forum here from those who have forged ahead and done it on their own. Then if you get that far with the physical hardware, there is the wiring issue and whether to use the factory tow computer module (huge mess, from what I hear) or do aftermarket wiring (simpler but no warranty support).
It is pretty much critical that folks buy these things configured how they want them from the start, adding literally ANYTHING after the purchase has been a huge expensive hassle for folks. A lot of people report their dealerships jerking them around acting like they can't even do a factory tow package install (because they don't wan't to). And they don't want to because they make no money whatsoever on that type of install.
They've got your money already, by the sounds of it, $600-$800 bucks is going to sound hella expensive to you to install factory towing kit but that won't barely even really cover their parts and labor, and now you are relying on the skill of your local tech monkeys vs buying one that had the entire factory tow package properly installed right from the factory. Its not just hardware, there is a trailer tow computer module that goes with and programming for the anti-sway and all that jazz.
You can install something aftermarket like mycampbell did but we've talked about that here on the forum as well, in a nutshell aftermarket is nowhere near as good as factory.
Unless your dealership service staff are really nice and you have plenty of money it might be a real uphill battle for you. Could get lucky, if they have not installed one themselves yet you can be the first, get a service writer to lock in a low price fast and early before they realize what they have to do and how many hours it will take them, maybe you will skate by but really really really should have gotten one with all the options you wanted/needed from the start.
Some reference links: