My car is having a problem of passenger side light (low beam) being always on even after turning the switch or engine off. I have to unplug it to prevent it from draining the battery. I am not too familiar with electrical system on the Prius, but suspect bad relay or something related. Anyone knows where the relay for the headlight is?
I don't think a relay would cause a single headlight to stay on, maybe both, are you sure the other low beam isn't burned out?