I did this to my 01 AE . Here are some helpful links on what you need to get for a aftermarket unit to work with your Bose system and steering wheel controls.
Scosche OEM Amplifier Adapter
http://www.crutchfield.com/p_142C4NN...n-Adapter.html
SWI-JACK interface from PAC
http://www.crutchfield.com/p_127SWIJ...K.html?tp=3405
Antenna Adapter
http://www.crutchfield.com/p_12040NI...a-Adapter.html
Your also going to need a Dash kit or some people go with a GXE Kit .