Willie:
I checked the instruction sheet at a local sporting goods store. I found the instructions were written in several languages and that they clearly stated a minimum of 3 square inches were required for safe operation and the unit only raises the temperature within the vehicle 20 degrees Fahrenheit. Unless you have a better way of trapping heat at 40 degrees below, you still have a temperature of below 0 F. Now having said all of that…How many people will actually read the instructions? Like you, I share that concern.
Stargazer

ASAP: Always Suspicious Always Prepared.