The regulator on the Focus is not internal to the alternator, and the dash illumination is controlled by the engine ECU. The Focus has what is called a "smart-charge" system where it only supplies the amount of juice needed, providing more power for the engine by not sapping it all with an alternator.

TBH, I would get it checked professionally, as when I worked for Fords, we were always getting Focus' in (mainly ST170 versions) and they could be anything from alternator, to ECU to main engine loom wiring.