What is this wire from the firewall bulkhead connector for? ( see pics)

I don't remember where it came from and couldn't find a place for it this evening. From the firewall bulkhead connector, its a single brown wire that is at the top, closest to the driver's fender. Anybody know what that goes to? I had looked at a 96 and thought I saw it, but when I got to my truck, it wasn't the same wire. Oh yeah, this is on a 97 C1500, 5.7 auto.

The wire appears to be a factory wire. It goes through the bulkhead connector in that silicone / rubber / whatever. I don't think this truck has ever been molested, until now.

EDIT: Oh yeah, it may have been taped to one of the larger groups of wires, or it may have wrapped down and under all the other wires coming out of the bulkhead connector. I have it pulled out to the relay center for taking the pictures.

It looks a little like the fuel pump test terminal, but on my '96 it is a red wire with black connector. Applying battery voltage to the test terminal will run the fuel pump with the engine off. Nothing connects to it, it just stays there until you need to check the pump, or for broken relay or relay control.

I wouldn't suspect they would put two connectors in the same area, and I don't think they would use a brown wire for the pump test terminal. So it probably isn't factory.

It definitely is factory, and it goes through the firewall and into the wiring bundle under the dash. A poster on another board said it is for a service tech to connect and run diagnostics on the ECM. Looking at wiring diagrams, that makes sense. It shows a brown wire with "To ECM" at the arrow on the end of the line, which I interpreted to mean that the wire went to the ECM, so I didn't think it was this loose one I have. It won't reach the ECM and there is no connector for it. I guess the wiring diagram means "To ECM test computer which then connects to the ECM".

I have one as well. I have no idea.