Druncle

Gidday team, I've just bought myself a 2006 Forester 2.0L tiptronic and installed a JVC KW-M750BT head unit into it. The head unit comes with wiring to connect to oem steering wheel controls and was wondering if anyone knows if I could link it into the steering wheel controls that are meant for the gearbox? My great end plan would be to wire it in a way that when the gear lever is in drive and the buttons don't normally do anything that they would control audio volume but then when put into tiptronic it only works for gears. I'm half figuring I could do this via a normally closed relay, there must be a sense wire to know when it's put into tiptronic, I had a bit of a delve around but could only find the up gear/down gear wires. Anyway, any thoughts on if it's possible, if it's been done before, how it should be wired, if I explained it badly, or if I'm on my own and I just report back when I f*** it up/make it would be great!