Cold Crack ratings are based off of the ASTM spec (D1790-94; Brittleness Temperature of Plastic Sheeting by Impact).
Points from the spec read:
- brittlenss temperature is derived where 80% of specimens would pass 95% of the time
- testing is based on statistical calculation from a number of specimens

The test is an Impact test. Which means something, like a hammer, is dropped against a part that has been in a freezer for an hour.
The rating then describes the limit of the plastic/vinyl.
So a -20 cold crack rated vinyl would not shatter at -20 if you sat on it in the car, but you're pushing the envelope. The more aggresively you jump into the car, and the colder it is, the more likely it will crack.
A few things to keep in mind.
- The inside of the car is usually not as cold as the outside airtemp.
- The plasticizers that are mixed into the vinyl to keep it from cracking naturally leave (propogate out) of the vinyl over time. So a -20 degree vinyl today will probably only be -5 rated material in 5 years.
- What is the coldest your area sees, and what is the coldest you would use the car?
- I wouldn't use anything that isn't rated at least 0 degree cold crack.

Hope that helps.