55.2-59 Wiring harness question

[55Trucker]

I discovered an unconnected grey wire coming out of the firewall, engine compartment side. I haven't been able to easily trace it on the cab side of the firewall, as it's in a taped bundle. It is in the ammeter harness, which seems to only have 2 wires (green, brown, going to the resistor) in the assembly book, not 3. It is grey and has "28" stamped on it. I called Lectric Limited, but they weren't able to help. Does anyone know what this wire is?