What is this blank below the head unit?

[spressomon]

^ I don't think Slee has the button covers today but that's where I sourced mine.

 

[Ayune]

Spressomon has done this. He says there is a twelve-volt off-on-on output, doing exactly what you want.
FYI: 12V switched power sources

Yea, I have rear heat already, my blank switch has On-Off functionality. The million dollar question is where does it go?