^^Good step 1 in the process of elimination. If your mirror doesn't dim in less than a minute, step 2 should be checking the connectors on the wiring going into the mirror, both for physical connection and for voltage (with the car running, a VOM should show nominal system voltage from around 12.6 to around 14 volts).
To go to step 3, you'll need to access Ford's online shop manual (since there isn't a commercial manual out for 2008+ models yet). Talk to your dealer's parts or service techs to get the wiring diagram you'll need to go on to test the connections further upstream.
All my relations, Christine
NW Posse #17
"Straight roads are for fast cars. Curvy roads are for fast drivers."