I would check the wire for being loose in the socket. I've noticed that on my 2014 Forester, sometimes the aftermarket LED lamp I put in there will ... seem a bit dim. I wiggle the wire and it's good as new ... at least until it happens again.
Or get to the local Subaru dealer and have them take a look. It will probably cost you about 60 bucks (or whatever the current hourly rate is for diagnosis) and you can find out the issue. From there, you can then decide what to do with the car and the repair needed.