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?

[55Trucker]

It looks like an under hood light was an option on these trucks. Since the dash bulb wires are grey, I wonder if this unconnected lead was for this option?

[_Ogre]

coolant sensor wire, 55 has a mechanical gauge, but the harness is probably 55-59
the end is similar to coolant sensor on the 56-59

[Divine 59]

I agree with Ogre- That connector looks like the type on that fits my coolant temp. sensor on my '59. The 55 2nd used a capillary tube style coolant temp set up.