The autodimmer, and any accessory lights on the mirror should work when you find the power and ground leads.
Onstar, a built in compass, good luck getting those to work properly.

If your windshield is missing the metal clip for the mirror, then you will need to get a universal mirror kit from autozone, napa, etc.
I would take the mirror to the store with you, to make sure it fits.

For pictures;
Google universal rear view mirror kit