auto dim mirror

I just bought a 96 2500/v10 Ram with the auto dim mirror, and it doesn't work. Very annoying on the drive home. Anyone have a diagram of how this thing works?
I think I'm going to hate feeding this thing, but man, is it an animal!

I bought a 99 truck in 2000 that had that mirror. It didn't work either, but if I opened the rear slider it did. The tint on the glass was enough to make it never work if the slider was closed.

Too fleepin' cold to open the slider!
But I will check that tonight. I think I will just change it for a manual version. I know they work when I want them too.