http://www.hhrvresource.com/VolvoDo...ke and Exhaust System Design and Function.pdf
Page 2 describes it a bit. If you have one equipped, it will only work when the coolant temps are below 50F. The icon is a yellow coil symbol on the dash. If you don't push the key to preheat the air before starting, it will automatically turn on when the truck is started if it's cold. It uses quite a bit of power, so make sure your batteries aren't low if you use it.
Attached are some pictures of the block heater on my truck.