Check your wire from the plug to the heater at the block. I have only seen one fail internally. The rest were broken wires. Continuity test and OHM out the plugs.
If you do indeed need to replace it, yup you will need to drain the coolant and prepare to take a bath.
Two ways to do this.
1. drive a self tapping screw into the old heater and pry it out or use a slide hammer.
2. using a punch, drive one edge in. It usually will spin the heater over then you can pry it out or pull it with a pair of pliers. If it drives it straight in, then it might get a little tricky. At least it will have something you can hang onto it with and pull it back out.