That's the fusible link. They were originally a specific length of wire at a specific gauge and out of specific materials so it would burn up if more than a certain load was placed on it. As you've noticed they do corrode significantly over time; They were replaced with self contained fuse looking versions, they're generally not called a fuse though. Toyota started using them in the MKIII toward the end of the '88 or '89 model year.
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