Odd electrical fault

[Lipinhu]

With the engine running and idling fine, you should get something arround 13/14V. From what you described, to me its all about the alternator.
1st- dont go and buy a new one, there are parts that are ment to be worn out after all these years, but you can esily repair it. My guess is the brush (or whats left of it) or the rectifier, once the stator doenst have much (if any) wear.

There are a repair kit at toyota (27362-18011, dont know how much costs) that brings the rectifier and the main brush. I think it will get you running like new. Another option its to look some junkyards arround and look for a Denso alternator with the same bolts place, there are lots of compatible and better ones.

I been in trouble with the alternator on other cars and allready checked it on my hachi, so...