there is a much easier way to perform this mod, i just did it 5 minutes ago.
No need to cut both wires from the side marker - black/white right side, black/red left side.
also no need to cut the red (blinker) wire and the green (parking light power) wires from the parking lights.
NOTE -The Below is FR-S Specific, as well I haven't tested this with LED bulbs, only incandescent. LED Bulbs in your blinkers WILL result in Hyperflash unless you have installed an LED safe flasher relay / Tapturn flasher.
I'd recommend using a multimeter or a test light, to verify the wires if you have a BRZ or BRZ Limited. The same would hold true if you have a 10 Series or Monogram, the standard FR-S wiring is what the OP's Guide and my revision are based on.
Here is what i did to achieve the same effect, took 5 min per side..
using the DIY in this post locate the red (blinker signal) wires coming from your parking lights and splice a jumper wire onto it, either by splice connection, solder, tape, wire nuts, whatever your preferred method is... do not cut the green wire or splice anything to it, the green wires are your constant power wires for the parking lights.
next, locate the 2 wires coming from your side markers. i chose to get my wires before the harness going into the sidemarkers, whereas the OP chose to get them inside the sidemarkers themselves, do it my way, its better because you have more room and wire to work with and easier to return to stock...for your referance - right side will be black/white, left side will be black/red.
The black wire on each side is the only wire that needs to be cut, NOT both wires going to the side marker. Again, for reference, the white wire on the right side, red wire on the left side is your constant power to the sidemarker, no need to cut or splice this one, leave it be. Yet i digress.. locate your black wires on each side of the sidemarkers, and cut the black wires. Take your jumper wire you spliced to the red blinker signal wire from the parking lights, and connect them to the sidemarker side of the black wires you just cut. tape off the remaining black wire ends that are leading to the engine side. if you ever want to reverse this mod, just splice the 2 ends of the black wires back together, and remove the splice connection to your red blinker wires.
test and enjoy
if something doesn't work, you either didn't make a good splice connection, or you ran your jumper to the end of the black wires that do not lead to the sidemarkers...
no hyper flash, no warning lights, blown fuses, just like the OP method above this flashes and looks stock, the way the fr-s/brz should have come.
1. Parking lights on, the side marker will stay on and alternate with the front blinkers.
2. Headlights on, same as #1.
3. All lights off, the side marker will not be on but will blink in sync with the front blinkers.