US and JDM SC AW11 used an external regulator.
MR2 alternators tend to be fairly durable, but do fail. You will probably note a loud "death whir" coming from the alternator for several miles before it fails. An idiot light should light up on the dash as well if the alternator has failed or is at reduced output. Often you'll see the 'three lights of death': the Charge, Brake, and Fan lights will come on, which normally indicates that the alternator or voltage regulator are bad.
In the USA, AutoZone will usually test the alternator for free. Alternately, this good general description on how to test the alternator might help with basic diagnosis.
Removing the Alternator
Replacing the MKI NA alternator is easy, two bolts and an electrical connection. The alternator is mounted with a long bolt on bottom and an adjusting bolt on top.
The MKI SC is much more difficult, as it was moved to the front of the engine to make room for the supercharger, and is above the AC compressor. To change the SC alternator requires either removal of the AC compressor, or it can be removed and reinstalled through the engine's air vent, also not easy. Either way is difficult.
- Check your alternator bracket bolts first, as this can also cause the 'three lights of death'. Check all ground connections.
- If you are able to get the alternator out of the car you should check and replace the worn and probably shortened brushes (if that's the fault) for about USD$4. Dismantling the alternator is pretty easy; replacing the brushes will require basic soldering skills. That might be the problem.