Question: Plug next to Speedo head

[cnefalcons]

Would someone please tell me what the plug is for next to the speedo head on a 1981.
I had the speedo out and this plug has one wire off of it. Is it a must have?
What is it for.
Thanks Mike

 

[Antz81]

Would someone please tell me what the plug is for next to the speedo head on a 1981.
I had the speedo out and this plug has one wire off of it. Is it a must have?
What is it for.
Thanks Mike

What colour is the wire?