Here's the steering wheel control adapter. Just make sure your aftermarket HU works with this kind of device.
Axxess Metra ASWC-1 $55.00 http://www.amazon.com/gp/product/B00B4PJC9K
Otherwise use the PAC SWI-X. That will work with any HU that has an infrared remote controller. I've got a used one I could sell you, if you go that route, since I'm switching to the ASWC-1.