The freezing point would be lowered from 0 °C to about -6 °C by the
alcohol alone. In addition, there are some sugars, salts, etc. which
will lower the freezing point a little more.
It's important to realize, however, that the wine will not all
freeze if cooled down to that freezing point. When some ice starts to
form, it contains almost pure water, leaving the remaining liquid with
an increased concentration of alcohol etc, and hence a lower freezing
point. Pure ethanol has a freezing point of around -117 °C. So even if
your car gets cold, you'll get wine slush, not solid wine ice.
Nonetheless, if it's good wine, you shouldn't be carrying it around
in the trunk because it might be damaged by heat. Also, if some does
freeze, the increased volume of the ice (water is weird that way) might
increase the pressure enough to slide the cork out some, and degrade
the seal. If you were more unlucky, perhaps the ice could plug the neck
and the bottle might crack.
(republished on 07/25/06)