I managed to find a picture of the instrument panel wiring harness showing that I3 male plug.
I've circled in red the connectors that match the plug. As you can see, there are 2 connectors with the same shape, one is designated by I1 and the other by I3 on the wiring diagram.
I beleive that the I1 is going somewhere below/left of the head unit, and that I3 is going somewhere behind the speed indicator or below the wheel.
Time to find that dash removal guide and hope for some sun, it's been raining forever lately.