I haven't found any information about using an iPhone 4S with my stock Pioneer unit but I think it's better than upgrading your stereo. With the iPhone, you press the on button and Siri (digital assistant) responds through the stereo system. You can tell Siri to call your wife or anything else and it works as if you had a system built into the car already and best of all Siri understands natural language, you don't have to learn the system, you already know how it works.
Additionally, I purchased a Mountek holder for my iPhone that grips the CD tray so that the phone can be used for directions, map reading or any other function you could want. Attached is an image of the setup. This is less expensive and in my opinion, far better than other solutions that I've seen.

 

You talking about using this as an alternative to an aftermarket head unit, or what?

 

Great info. I had a jeep before this and the Siri integration was garbage.

 

I'm saying that with an iPhone 4S you don't really need anything else and as a bonus you don't have to learn another system which an aftermarket or upgraded stereo would entail. The stock Pioneer stereo that comes with the Scion tC is all you need. When I looked at other new cars, the Cruze and the Focus, for example, they had this system that you had to use for making calls. There's nothing wrong with them but since I have the iPhone 4S and Siri is very easy to use, I didn't want to use another system for making phone calls. I think this is superior to other systems. It's a personal choice but I wanted people to know how it works because there is no published information on this topic, until now...