I have not worked on a model of that vintage but this is how it is done today. There is a plastic air intake grill on the front top of the unit. Inspect it closely and you should see at least two screw holding on the grill. Sometimes the screws are between the louvers about an inch inside. Once you remove the plastic grill you should see two more screws, one upper right and one upper left. These screws are very long and go all the way to the mounting bracket on the wall. Remove these screws and you should be able to lift up on the back of the unit and lift it off the bracket. Also, there really should be two screws going down through the floor of the cabinet into the unit.