Can you let us know the exact type of alternator please. I'm hoping its the common Lucas ACR type and not a delco one from a chevette or something like that. The Lucas has a fully built in regulator, the delco needs an external regulator........
To wire a Lucas ACR, the two large terminals go to the battery Positive terminal. The small wire needs to have 12V fed to it to energise the alternator. This is provided by the following circuit. Ignition switch engine run position sends +12V to the ignition light (red), then from the second terminal on the bulb to the small terminal on the alternator. This way, when the alternator is not charging, the current flows from the ignition switch, thru the bulb to earth thru the alternator, so making the lamp come on. When the alternator starts to charge, the small terminal becomes the same voltage as the battery, so no current flow thru the bulb and it goes out. Its as simple as that.
FYI, a faulty alternator can often be diagnosed with the lamp, dull glows mean either too many volts, or to few volts so that current flows one way or the other thru the bulb making it glow.
Picture of an ACR here:
http://cgi.ebay.co.uk/Rebuilt-Lucas-17- ... dZViewItem
Cheers
Tony (HA21)