I have a 1991 Miata. It's a Canadian vehicle, and so is equipped with daytime running lights. Recently, I've noticed that the passenger side light is not working as a DRL. The driver's side light works fine. A multi-meter shows that no current is being delivered to the passenger-side bulb, while the driver's side get 14 V.
The turn signal bulb is used as the daylight running light. The passenger side light works in all other respects. When indicating a right-hand turn, it blinks. When the main lights are on, it's on. When the driving lights are on, it's on.
I suspect that the relay responsible for turning on the daytime running lights is failing, but before I order the part, I thought I'd check here if there are other possible explanations for this behaviour.
Some more information in response to questions in the comments:
The bulb for the DRL is dual filament. The brighter filament is for the DRL and the turn signal. The less bright filament lights when the headlights are on.
When DRL are on, the turn signal sequence is off-on-off-on because the DRL and turn signal use the same filament.
The wires to the connector are red, green and black.
And one piece of interesting new information. The passenger side turn signal (right-hand turn) is starting to act strange intermittently. The relay cycles faster, maybe double the normal speed. The front turn signal does not light at all, while the rear on acts normally. This is the behaviour I've seen before when the front indicator bulb has burned out.
I'm starting to think the problem is not the DRL relay, but rather an electrical fault somewhere between the relay and the bulb. Since the turn signal and the DRL use the same filament but different relays, the fact that both are failing makes me think there's a problem in the wiring downstream of the relays.