Do tefilin get ruined if left in a freezing car or that a myth?
It depends on the ink. In the cold everything shrinks, however the parchment and ink shrink at different speeds. When it warms up again, the parchment and ink expand again, at their own speeds. This won't for sure make the ink crack, but it sure helps. Especially if it gets warmed and cooled a lot. Think bending a plastic cc back and forth, it will most probably snap at some stage.
There is an ink called דיו לנצח (dio lanetzach) which has more elasticity and would react more like a rubber band being stretched and released, and would therefore not crack.