i was wondering if it's possible to hook up the steering wheel radio controller to an aftermarket head unit.

Its pretty damn hard, but I think some people have been able to do it with the Avic-D3 you have to get a kit to make it work and you have to solder some resistors...

you can buy an adaptor from pac

itsl ike 70 bucks and is very easy to hook up

look on crutchfield.com

put in the car make/model
and radio make and model

very easy to do

search for pac-swi

it'll only work if the aftermarket headunit even has the option to be controlled by an external source.

Squall is right. You need to have a radio that is capable of it. The pac-swi is mainly for sony or pioneer. They recently released them for alpine and etc. It is a little bit of a pain but all the wires are right behind the radio so dont get flustered thinking you need to go all the way to the steering wheel.

Some of the eclipse units will allow you to run the steering wheel controls straight into the unit. The CD3200 and CD7200mkII in particular support it I believe. They have downloadable profiles to load onto the head unit so it knows what each wire does.

My CD7200mkII is on the way, so I'll let you know if I figure it out and if it works :)