This may seem like a dumb question, but I've had multiple friends call me who were following me to tell me my taillights were out. I changed all the bulbs and verified that each bulb illuminates:

 

turn signal light

reverse light

brake light.

 

I think the same bulb that operates as the brake light operates the running lights. It's a 2-fillament bulb and one maybe illuminates as the running light and 2 as the brake light.

 

I took it into the dealership and they said nothing was wrong with them. It's just frustrating b/c I can't really see behind me when I'm driving at night so I don't really know what's going on back there.

 

My question for anyone w/ an outback sedan, model 2005 or later is, are your running taillights pretty dim??

Try parking your car rear-end to a wall, preferably after dark.

Then press the brake pedal and/or put the parking lights on.

You should be able to see reflections of the lights.

You can also do this when parked in front of another car.

Either way, you can check whether your lights are coming on.