I wanna know if it's possible to [turn on] the stock stereo from the steering wheel's tri-btn config (UP, DOWN, SKIP).

 

I've never seen it done. Which doesn't mean it's not possible just not sure the benefit and whether it's worth the work.

 

Anything is possible when you're wiring a circuit

 

You could just leave the stereo on and it will turn on with the key. Wouldn't that be easier?

 

Lazy man answer: Leave the radio on, and just turn it down all the way when you're not using it, then use the volume controls to turn it "back on"

But it would be cool to somehow make it so you hold the Skip button to turn the radio on and off; that seems like the most reasonable way to do it.