OK, I'll give it a try. Reference page 61 of the electrical manual.

First, remember that the engine MUST be running in order for the rear defroster to turn on. If the engine is off it won't turn on.

When you hit the rear defrost switch can you hear the rear defrost relay click and does the rear defrost light come on and stay on? Yes? No?

If no click then the trouble is in the circuit that controls the relay coil. The blue wire at pin 1 of the coil should have +12 volts. If the relay clicks and stays on the blue/red wire at pin 3 should be 0 volts or very near 0 volts. If no click and pin 3 has +12 volts there is the problem. The alarm module, for whatever reason, is not allowing the turn on. Verify that the ground wire between pin 10 of the rear defrost switch to ground is good. Make sure you have continuity.

This should get you started. Obviously not seeing the actual symptoms is gonna make this hit or miss but we might get lucky.