Voltage isn't an absolute diagnosis. I'd pull the alternator apart, check the collars for excess wear and replace the brushes. At idle the electrical system may run at a deficit due to the small size of the alternator and age of the overall system... It's a fairly common complaint and the battery should compensate for the most part. Some people swap in an alternator from a turbo MR-2 which is 100A or 110A... I think it's just 100A (I'd go look at mine but it's hell to get at on the MR-2.
Another thing which can cause headache is old style, wire type fusible which have begun to degrade. These were used regularly up until 87 or 88 and then intermittently (until Toyota ran out I guess) into even 90's models. If you look in the engine compartment fuse box and the 100A fusible link doesn't look like a large plastic fuse but rather just a large piece of wire you should try replacing it with a new one (you can use a sealed style one, it bolts right in).
Then just start pulling and cleaning all of the major cable connections.
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