In case it helps later on. On my car the small harness which contains that wire begins at a plug close to the firewall in the centre of the engine V. It then splits in Y with one leg going to what l think is vacuum valve low down at rear of RH intake manifold and other leg is wire you have which on mine goes on to supplimenary air valve.

 

From what I can see on the photo it isn't a solid brown wire. Their appear to be dots of colour along it, rather than the old style tracer stripe. Narrow down that colour and you can determine function.

Could be supplemental air valve, cruise dump valve or something going to another harness. I'm not familiar with the European emissions components, but there were a number of solenoids etc.in that area.

 

The wire is brown with slate trace rings spaced at about 40mm along it's length.
It is shown in wiring diagrams (81-88 engine performance pdf) as connecting to 45 second timer via plug CP8 and running to vacuum solenoid valve, supplimentary air valve and other options depending on market requirements.
What is strange is the fact it seems to be present on some cars and not others even though they are the same MY.

 

I looked at the engine performance info and it looks like there is a NS wire to the vacuum valve (which my car does not have) this is connected to the air injection and vacuum advance. Your car looks like a Marelli from that coil in the left of the pic, but if it had/has air injection I suspect this wire goes to the vacuum valve solenoid.