I'm really not sure about the 'free installation' offer. If they're running a special, the harness and dash kit "should" be included in the install, you just have to pay for the parts.
In my case I had the new Geek Squad coverage that covers installs and repairs (free), for every electronic device in the store. That included the car stereo, dash kit and harness in my case. You might have to buy this coverage, unless they're just running a 'special' on car stereo installs at the moment.
I'm sorry I can't answer your question more definitively. I suggest that calling your local Best Buy will give you a more direct answer.