Henry, I checked the schematic; there is a relay (N10) that controls the wiper motor, turn signals, and rear defroster. The black wire on terminal 6 (pin87) goes to the rear defroster.
The wife has the car today so I can't check; but I assume the relay is in the main fuse box near the brake booster.

Maybe this will get you started.
1) Check to see if the wipers and turn signals work.
2) If they do, pull the relay and try the wipers/signals again. this confirms you have the right relay.
3. Lift the relay socket up and jump a meter onto the pin 1-30 (red/grn wire). This should be hot, coming from the fuse. Then check pin 6-87 black wire (with the rear defroster turned on). This is the wire going directly to the rear window.
4. if you get voltage on 6-87 then the relay is good, the problem is in the wiring to the window, or the window resistors themselves. (I'll bet it's a broken strip in the heater).
5. If you don't get voltage at pin 6-87, check the wires from the rear defroster switch (pin 3-blue, pin 5-black). But since the switch lights up, it's probably sending voltage to the the relay coil).

The fuse for the rear window is a 30 amp, position #3, with a red/grn wire.
You checked this, but might put a meter on it also.

Hope this helps.

DG