The switch I'm referring to is a "soft" key, the function is listed on the blue screen, the hard switch below the screen is what is pressed, it's on the right, maybe not the extreme right.
With the headlights off the switch makes the screen even brighter in case the sun is shining more or less "on" the screen. It works pretty good, i've had to use it before.

I may be able to get ahold of an MCS wiring diagram later this week.

Gilly