The light bulb (and socket) that comes with the OEM foglight is a single element bulb (meaning it only illuminates at one level of brightness). You would need to locate a socket and bulb that would fit into the OEM unit, and then tap a parking light wire for the "dim" illumination, and a brake light wire for the "bright" illumination. I don't know how easy/hard it would be to locate one but you could take your light assembly to a local Pep Boys, Autozone, etc. and see if they carry anything that would support a dual filament bulb and fit the light.
A search using "370Z LED fog light" yields:
EVO-R INC. Nissan 370Z Z34 LED Rear Foglight
Good luck,
Dave