I'll be doing this mod in the next couple of days, it's worth pointing out that you're more likely to NOT have cruise control if you have a GT86 as apposed to an FRS/BRZ, very few JDM MT 86's have cruise control.
The switch itself is used on dozens of Toyota vehicles, the Toyota part number (Different from the number in OP) is 84632-34011, and pretty much any harness for this switch for a toyota vehicle will work, the only difference is the roll connector side of the harness will have (most likely) a 4 pin connector meant for the prius or similar, you can simply disconnect the 10 pin roll connector side of the connector everyone has installed by default, and plug in your black and red lines from the cruise control switch.
Most of the RAV4, Prius, Yaris, Corolla, TC, xB and xD vehicles use this exacts same switch and can be snagged on ebay for as little as $10 new.
Just make sure it looks like this:
Here's a real basic view of where to wire the switch.
I've checked and my non-cruise control JDM GT86 does have CSS wired up to the roll connector, as with aagun i'd be interested to know if anyone actually had a model of the frs/brz/86 where CSS wasn't actually connected / was missing from the harness entirely.
I do also wonder if the generic 4 pin harness would work plugged in to the vacant slot on the far right, too? will test when the switch arrives i guess.