I don't know if you are still concerned about this, but...You're right, I wouldn't necessarily know which light was out, but at least I would know whether driver or passenger side. Does the new lamp alert specify which bulb of the possible three? Honest question.
If the turn signal indicator on your dash flashes very fast when you put on your turn signal -- indicating a bulb out -- there's a special procedure for figuring out which bulb is the problem.
But it's way too complicated to explain here...
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