Rear window defroster light.?

hello i noticed today that when i went to turn on the rear defrost knob in my 91 olds 88 royale the little yellow light didnt come on, however early this morning on my way to work i turned it on and the light was on. it rained thru out the day, but since i never use the back seat area ever, and i never put stuff near the window how can it all of a sudden not light up? I did install a new car stereo late last week but i used a good universal wiring harness with it and the stereo wires in the car were very isolated from any other wires so is it possible that the light bulb went out in the dash and the defroster still works, or does it mean that it needs service. any advise or help is appreciated