Adding aux controls like the steering wheel?

I am wondering if I could add auxiliary controls like the steering wheel on my 08 Mariner. I have the 2 piece 6 disc radio but do not have steering wheel controls. I would like to add additional controls for the radio somewhere maybe the console or door panel. Is there a plug in harness I can get so I can add this? Is the switch input just a - or + or something special.
I want to add other options to my mariner as time passes.

Thanks

Tim