Takes the place of the OEM flasher unit. You'd pull out the OEM unit and plug this in so the flashers don't blink a million miles and hour due to having the lower resistance of LED bulbs installed.
Don't know where it is on the 4th gen but on my 3rd gen it was located on the back of the fuse block. Assuming this one is in the same place, you have to remove the lower dash panel, unbolt the fuse block and pull it out.