Here in MN, many vehicles used to come equipped from the dealer with a block heater - the end of the cord is left dangling out the front grill a few inches. I think they are only 50 to 100 watts or so. Makes a huge difference in start-ability in cold weather if you plug it in a few hours before you want to use the vehicle.
Some commercial and government buildings are even equipped with rows of outlets in the parking areas for the employees to plug in the heaters during the day so their cars start at night. Also common in fleet parking yards for school buses, police cars etc that _need_ to start reliably the next morning.
I have personally used a magnetic stick-on heater on an old car that had a weak battery on a one-time basis. It worked great. However, I wouldn't trust the magnet to hold it on while you drive - a chunk of ice or something could knock it off of the oil pan.
I have also witnessed a buddy using a pan full of glowing charcoal slid under his car. That heated it up and got it started in less than 20 minutes. However, the charcoal technique is frowned upon as being fairly unsafe because of the ease of which any leaking or accumulated oil on the outside of the engine can be ignited!
Modern fuel-injected-into-the-cylinder cars and better low-temperature performing oils have made such tactics generally unecessary with newer vehicles.
One of the issues with carburated engines in cold weather is that a lot of the fuel will condense on the inside of the chilly intake manifold and cylinder walls and will not form the proper air-fuel mixture for good combustion.
- Rick