What I'm going to post is not going to be looked at favorably by the OP, but as the ex bodyshop owner on the forum, what I am seeing appear to be "deflection cracks", they are not a factory defect, something came in contact with the taillights causing them to flex inwards to the point of cracking. It could be as simple as someone kicking them (yes, I know how high up they are), or backing into a bush or two (this type of cracking I have seen before in just this type of situation), how this happened may remain a mystery, but somewhere along the line, it happened.
Bill S.