This is pretty easy to fix. What happens is the connector to the black box gets hot...it isn't that good of a connector to begin with. Anyway, the connector heats up, and melts the plastic connector going into the box. When the plastic melts, the pins separate, and the connection is even worse, so it gets even hotter. Eventually they separate enough that the connection is lost and the defroster doesn't work at all.
There isn't a problem with the black box though! If the terminals are melted a little it probably doesn't need to be replaced. I just cut the (ruined) connector off, took the black box apart, and soldered the green and black wires directly to the pins inside. It doesn't take too long, and doesn't cost anything, and that connection won't fail again.
The reason it fails again for some people is that the connector is ruined the first time it happens. Even with a new box, the connection will be bad if it works at all, the terminals will heat up again and you still have the same problem. At least replace the connector if you are going to try to fix this. If you solder it, make sure you get a good connection on the larger black wire, there will be a lot of current going through it for the defroster...that is why the connector melts in the first place.
The box is located in the pillar between the passenger side window and the rear window...right below the two defroster connectors on the passenger side of the rear window. Those two wires go into this box, about six inches away. You'll have to remove the three panels in the rear on the passenger side...I was working on a two-door Alero so a four door might be different. I had to remove the passenger side seatbelt hoop and the coat hanger hook in the rear. You need a large torx socket for the seatbelt, and a flathead screwdriver for the coat hangar.
If you still have any questions let me know...it isn't hard to fix. It took maybe an hour, and I didn't really even know what I was looking for when I started.
steve@wotelectronics.com