You have two sockets in one housing, correct? Are you dead certain that both sockets are grounding to that housing? I'd be willing to bet you're not closing the circuit in the dual-filament socket; try poking the side of the bulb with a grounded test light when the pigtails are hot. If the bulb lights up, it's a bad ground. If not, perhaps the contact points on the pigtail are worn or broken so they don't touch the bulb at all? Perhaps the socket is worn where the bulb's retaining pins should seat?
