Assuming you want to use the existing Bose speakers and amp and just replacie the head unit, you need a special amp intergration adapter. It's available from www.crutchfield.com
and other vendors.
If you're going with all new equipment, you're going to need to bypass the stock Bose amp, which I think is located in the passenger side das. The harness you need is actually plugged into the stock amp.
Read the C4 Stereo Sticky for all the details.