I just bought a 2007 MMH. I have read through the manual, through the FAQ and searched the forum. I've searched the Internet and even had others look through the manual.

WHAT is the button, under the radio and just to the left of the Passenger Air Bag Off Indicator light. It's a blank spot in the manual and in the quick reference quide showing a diagram of the dash controls.

 

On my 09 MMH, that button icon (button is in a different place in my vehicle) stops the beep you get from the sensors in the rear bumper that tell you when you getting close to backing into something.

Note that is is not a permanent on/off button, it just stops the beeping once it starts (you close enough to start it).

Edit: Here's the write up from the '09 Manual:
And in the illustration in my manual, it shows the same things on the icon (the triangular traffic cone, and the radio waves, but the waves are coming from an "R", not from a "P" like you've shown in your pic.).

 

Thanks. I finally figured it out by accident a few days later.

Really like the Mariner so far. Averaging about 33mpg's in town and still learning how to drive it. Only 2 tanks so far.