From what I'm told it suppose to turn on the parking lights with it beings to get dark, once dark it will turn on the headlight. I've always seen the 5 gen version to the auto headlights but never a 4th gen specific, the harness is direct plug and play.
It's JDM toy. And it was available only in Japanese 4th gens. It will automatically switch on your headlights. There are two steps and it will switch on only parking lights and when it get darker outside then it will turn on low beam lights. I'm not sure what does switch do. It was sticked on left side of dashboard just behind the windscreen.
Anyway where You found it? I would like to buy it if it's possible and join it into my jdm collection