Three wire alternator on a two wire system?

Most parts store regulators may look like "mechanical" but they will in fact internally be solid state. Easy to tell Look at the ebay auction of the underside

On older mechanical units, there is two large wirewound resistors on the underside.

I see what you're saying. I guess its possible that the one on my car already may work (I'm not sure its original). I'll have to take a look at it when I get home.