I hooked up the dimmer wire, and had the same concerns few others had expressed. The unit didn't appear to be dimming the screen at night with the lights on. 
My eyes.. it hurts them...
Apparently the screen bightness has two modes, day & night. Set the screen brightness with the lights off for day mode. Exit out of the setup menus back to the top. Then turn your lights on and go back in and do it for night mode. Exit back out.
Now the screen dims when you turn the lights on. Not drastic differneces, but enough for me.
I may just be kinda slow. If everyone else already figured this out, please ignore. If not, good luck. Worked for me anyway. 