Why my 2000 Honder Civic's front signal light does not work when I turn on the head light. But if works when I use as signal light?

The bulb has two filaments in it. 1. The parking light filament might be burned out. 2. The ground connection to the bulb might be bad (corroded socket?). This will make that signal light flash oppositely to the other signals when the headlights are on.