Check out www.crutchfield.com
they are awesome for this stuff. If you tell them exactly what you have(factory sub and or blaupunkt radio) they can get you the correct harnesses.
Unfortunately to keep the steering controls I believe you have to buy a unit that links them to the non OEM head unit. I did this and they work just as they did with the factory radio. It was like $50 which sucks but its definitely worth it.
Hope this help, or at least points you in the right direction.