I am currently working on my wiring - i checked it and from what i see that wire is power windows related, it is on the same loom, just after windows relays & circuit breakers. I measured it with multimeter & it goes from relay, this wire is also related to additonal square plug (marked on photo).
Maybe @sfdon could share some info?
From what I remember on my 76 E3, I think (again think), that wire was plugged in to the driver door light switch that had two terminals. It would allow the windows to operate once the door was open and ignition off. It may have
been an option on 74 and later cars!
The black and blue wires in kick panel are window motors wires. With two wires this must be a mid 73 and later with circuit breakers. When door switch is opened the load shed relay closes and allows windows to operate without the ignition. Door switch will have two wires as one goes to the load shed relay. If you push it in you can hear relay click. As the wire looks to be brown which is ground I think it goes to door switch. If it has power then you have other issues.
Brown wire might have battery voltage on it if it is connected to the bottom of the relay coil.
I would check with the bulb tester first and - as a second step - connect it to chassis. IF the relay will "tick" it can be "comfort" functionality of the electric windows (to close them with open doors)
I thank you all for the useful info! Seems like it relates some windows function because my driver's door switch has two terminals. The problem is: if i touch it to the chassis i have my ignition on. Even with no key in.