Fixing Rear Defroster on a 240

The switch behavior is normal, '83 and newer 240's have a timer switch on the rear demister so you press the switch and the light should come on and stay on for a set amount of time. If the light doesn't come on the first thing I would check is the fuse, even if the fuse is good they often get corrosion on the ends. Also you didn't specify whether this is a sedan or a wagon, if it's a wagon it's also *very* common for the wiring that runs through the tailgate hinges to break from the repeated flexing. Also sometimes the connector that plugs into the back of the switch develops some resistance from oxide buildup which causes it to heat up and melt, sometimes to the point that the wire falls out.