[notmytarga]

This was stated on the listing for the Alpine at Crutchfield:

Want to keep your steering wheel controls?
In most vehicles, you can use an adapter to connect your vehicle's steering wheel controls to this Alpine stereo. Connect a few wires, then program the adapter for your particular car and the Alpine, and you'll retain the convenience of your steering wheel controls. Check the "Accessories" tab for compatible adapters.

Steering Wheel Audio Control Adapters - http://www.crutchfield.com/Learn/steering-wheel-control-adapters.html
If you're considering replacing your factory radio with an aftermarket receiver, adding a steering wheel control adapter lets you keep the full functionality and convenience of your steering wheel controls. In this article, we'll cover how steering wheel adapters work and how to choose one that...

I've been thinking of putting one of these in the Targa.