They say the sonar modules will not work under 50 degrees Fahrenheit (10 degrees Celsius). I have tried using the 4, 10 ohm resistors but this seems to work much better, and is easier to install for me This is how I keep my sonar module warm in the winter, all you need is a small piece of the strip of red LED's, this is where I got mine
make sure to get red, they are much hotter than other colors. (red LED's usually use more power (watts) than other colors)
you only need a piece with 3 leds o it, it wraps around the sonar perfectly (currently cost .46 cents)
Just remove the film for the adhesive and wrap around your sonar, solder the wires to 12v and use a tie wrap to hold it securely. I hooked mine into my lighting so I can switch them on and off.