Time to bust out the wiring diagram and the test light... Electrical work's easy, just don't be intimidated. If you don't have the wiring diagram or a test light, buy them now. The wiring diagram is maybe $20.- (20 minutes of your professional mechanic's time), a test light is >$2 at Radio Shack. Both will pay for themselves over and over again.
Getting at the heater fan motor's a PITA, I bet, but the rear defroster's a piece of cake...
Switch it on and see if you've got current back there by connecting the test light to the connectors at the opposite sides of the window. If the test light comes on, then the grid's bad, and you're screwed, because there's no good fix except replacing the rear window. If it doesn't, start figuring out why... check the fuse (again), replace your perfectly fine fuse with another one, then check the relay, check the switch, so on so on, you get the idea.
The heater fan motor's bound to be more difficult, because you'll have to figure out some way to get at it... better to just figure out where the wires are and see if you've got current to the motor, could be it's burned out. More likely, it's not getting power because you've got a bad fuse/relay/switch somewhere.