I originally posted this on the DS3 forum, but thought I would repost it here.
I bought the hitch for my car on the web. It may have been from hitchusa.com. The dealer wanted $700. The local hitch place $500. Bought it online for $175. You can look up the hitches by make, model and year of car. The also post install instructions so you can decide if you are up to the install. The instructions for mine were remove four bolts, put hitch in place, then thread in four bolts. 15 minutes
The front hitch I installed on my in-laws' Tahoe was more complex and I had to remove the front trim panel. Two hours.
The wiring harness from the dealer may be worth paying for. I have had a lot of trouble when my trailer wires were spliced into the tail lights.