Tearing Tzitzit in Mourning

Is there an obligation to tear tzitzit, and is there a difference in the law between someone who wears it over the shirt and someone who wears it under the shirt?

There is no need to tear a tallit katan. There is no difference where it is worn since it is an object of a mitzvah.

In the book 'Meshiv Davar' (by the Netziv) Vol. 2, Siman 89, it is written that one is not obligated to tear a tallit katan, as it is worn only for the mitzvah. In the book 'Binyan Yehoshua' (on minor tractates), it concludes that one is obligated to tear the tallit katan, and it is said that the Chazon Ish was uncertain about this. Practically, the poskim write that one should not tear the tallit katan, as it is a garment for a mitzvah. (And even though the mitzvah is to attach tzitzit to a garment worn for wearing, and the garment itself has no sanctity, nevertheless, since it is not worn as a garment but for the mitzvah, there is no obligation to tear, as we find that a garment made for honor and rarely worn is not torn, as the Rema writes in paragraph 10, and likewise, a garment made for sweat is not considered a garment to tear, so too a garment worn not for wearing but to fulfill the mitzvah of tzitzit).