it will be the tacho feed wire which is run off the alternator. Either the tab where the wire connects to the alternator needs cleaning or the connector where it joins the main loom.
The alternator feed is the right hand of the connections here:-
Mine had a spade connector onto the alternator and was a poor connection, so I cut that off and fitted a ring connector then used the terminal bolt to bolt it direct to the alternator.
I then re-made the standard loom connection (another male/female spade) to ensure a good connection and that solved it.
You can see the spade connection to the alternator on the white wire here:-
The thick brown wire is the feed to the battery and the thin brown wire with the red wire coming off it is the control wire for the charge light on the dash. Ignore the red wire - that is the trigger for my split charge relay.
