Adding steering wheel audio control to MK1 P2

[Kennymen]

I own a Yaris MK1 P2 with the Matsushita CD stereo.
I would like to have the steering wheel controls too, so I was thinking to get a second hand original switch and build it in.
Do I need the whole wiring from steering wheel to unit (as far as I know the controls are connected to the Multi-display assy) or just the controls?
Or is it even not possible to add controls?

I was thinking to get something like this: Yaris second hand switch

BTW: I'm also thinking to build in a CD-changer MP3 extension to play music from SD/USB. Does anyone own something similar?
USB/SD/AUX for Yaris 1999-2005

Thanks, Kenny