Anyone know what this plug does?

[Nakilad]

It looks like the block with 4 terminals (second from left if you like) is the diodes for the rear defog and the tail lights, the one with 2 terminals is the diode for the heater (second from right), both of which are part of the idle up system (they energise the idle up vsv, but don't seem to directly interact with the idle up computer itself).

If that is the case, the only other items that should be in the same are as those (according to the wiring routing) is a diode for the headlamp / rear fog lamp and the rear fog light relay. I guess from your signature that this is a kiwi loom, and since my diagrams say that rear fogs were only fitted to european cars that might explain why i can't see a relay plug in your picture there.

It does look like there are two green / white wires coming out of one of those terminals (your marked plug), one of which is passing to the other plug (second from right), and maybe also over to the third one (second from left) and i don't see any wire of that color in the idle up diagram (that's no real surprise though, these diagrams are notoriously bad for wire colors).

Sorry, maybe not as much help as i thought i would be there!