Step 1: Check for a blown fuse in the fuse box.
Step 2: If fuse if ok, start the car and turn on the defogger switch. Use a volt/ohm meter and check for continuity between the -ve terminal of the defogger wire on the rear window and ground (find a proper grounding point, not a painted surface). If there is continuity then the -ve wire of the circuit is ok, otherwise there is a break somewhere.
Step 3: Use the meter and connect one end of the meter to the +ve terminal of the defogger wire on the rear window. Connect the other end to ground. If there is no voltage, there is a break somewhere in the +ve wire of the circuit.
The +ve and -ve terminals are not labeled so you'll just have to check both and use common sense to determine which one is which.
If there is no break in the circuit, either the defogger switch or the defogger relay is faulty. If there is a break, you'll have one hell of a job finding the source of the break
If none of the above makes sense, consult a professional electrician.