I have one in the M6, you need a SIM card in it. They usually have a back up battery in them but need to be connected to the car battery. The one I have has a high enough current draw, 100mA. This is the thing you need to watch out for on them.

You can text this unit lots of commands, like address, multi updates over time, over speed alert, movement alert, direction change alert, etc.
There is also an online Realtime tracking website, I haven't registered for that yet though.