Unknown wire at D13 on C1 ECM plug

Can anyone tell me: The C1 ECM (larger plug) on my 87 GN has an unknown wire in it at the D13 position. According to the schematic I have this position is supposed to be empty. It is definitely not factory installed and is not run with the regular harness and it is not hooked up to anything.

What is or would it be used for?

Far as I can tell it has only ever had the factory ECM in it and no other related modifications.
Thanks in advance for the help here.