how hard would it be to adapt the factory steering wheel remotes to work with an aftermarket headunit

how hard would it be to adapt the factory steering wheel remotes to work with an aftermarket headunit
There are products out there that will interface between the factory remote and the remote for
the HU. Do a search in this forum. I think its be covered already

Go here..........this unit works

http://www.virtualoutlook.net/Products/steer.htm