Is there any harm in leaving board games out in a car during cold weather?
Will any harm come to board games that I leave out in a car in that ['below -18°C/0°F'] temperature?


No.

I have had some board games for 30+ years that, during that time, spent two years in a Minnesota storage garage and three years in an Upstate New York storage garage; in which neither had heat or air conditioning (neither location was ever particularly hot during the Summer).

Some games had adhesive tape repairs; some had a clear adhesive film protecting the graphics of the gameboard.  None exhibited any damage due to storage conditions.  

Note: For long-term storage, all games are placed within plastic garbage bags which, in turn, are put inside plastic totes.

On one occasion, I noted one factory shrink-wrapped game developed a some minor condensation between the shrink-wrap and game box.  The game box lid was concave from being pressed; causing space between the shrink-wrap and box.  The condensate was noted when I brought this game home, from storage, during cold outside temps. It did not appear to damage or stain the cover of the gamebox.