and I quote: "Cold. Bedbugs can tolerate temperatures below freezing but cannot survive extreme cold for long periods of time. Freezing small hard to treat items or furniture will require temperatures around 00F for 3-4 days. Bed bugs that have been exposed to cold prior to putting in the freezer are more tolerant of the cold."
Again, it does not talk specifically about bed bugs in cars, but it points out that your car must meet the temperatures requirements for a certain length of time in order for the bed bugs to succumb. Same for luggage, baggage or any other item you want to protect from bed bugs with a DIY cold treatment.