Swap ballasts. If problem stay on the same side, it's the bulb. If problem moves to the other side, it's the ballast.
... or ...
Find somebody dumb enough to let you try the bulb in their car. If bulb works, you have a bad ballast. ...
... or ...
The bulb and the ballast could both be bad.
Not sure, but I think the ballast is covered under warranty but bulbs are considered a "normal wear item."
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