Hello one and all

I have a 1.9l WBX 1984 with a 90A Alternator.

The MYSTERY: there is a wire with a small round four pronged (male) plug
(larger round than a pencil, smaller that a dime) mounted on top of the
alternator (it ends at the the plug, that is not connected to anything else,
prongs inside rubber sleeve pointing to driver's side).  This wire comes
from beneath the passenger side air intakes and I can't follow it any
further.  It APPEARS to go into the crankcase at the juction with the
passenger side cylinder bloc, but that can't be right.

I can't find it in Bently.

Any ideas as to what this wire is or to what it should or might be connected
to?  Is it maybe for an option I don't have?  Diagnostic tool connection?

Thanks in advance for any input.

JGS