David
It sounds to me like that wire is a voltage sensing wire and is like connected to or near the chassis battery positive .
On that particular alternator there is a cap which covers the back of the regular and brush slip rings ,out of each side of that cover there should be a wire .
One runs to the ground stud on the back of the alternator known as( B-) or battery negative, that stud should be connected to a good battery ground
Ideally the battery negative terminal but most often to the rear ground stud of the starter.
The other small wire which exits the rear of the regular cover is usually connected to the ( B+) or battery positive terminal stud of the alternator
This small wire is providing the battery sense voltage for the regular allowing the regular to regulate its output accordingly.
The problem with this type of setup is that any volt drop ( caused by poor connections long wire runs or a Diode type battery isolator as where used in earlier coaches )Would cause the regular to effectively see its own output and not true battery voltage resulting in under charged batteries.
Because these coaches often have their battery a distance from the alternator the length of the battery cables and the afore mentioned volt drop associated
Some of these coaches where set up with an external sensing wire to give the alternator a more accurate battery voltage to work towards.
On this setup the small usually red wire running out of the right rear of the regular cover to the B+ terminal stud is supplemented with a small stud on the top of the regular where the alternator receives it's battery voltage sensing usually marke with an "S"
Although this bulletin is not for your particular model it is close enough to give you the layout.
https://studylib.net/doc/18030815/universal-alternator-wiring-diagramsHope this helps
Eric
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