Why would the rear defogger switch stay on for a 1996 Buick century?

Answer 1

As most vehicle defogger controls operate in a similar manner, and as I'm not familiar with your make and model, this answer will be generic in nature. Rear defoggers generally draw a fairly large amount of current, and therefore that current does not pass through the cabin control switch. That switch controls a special type of relay [a time delay relay]. When you press the switch to turn the defogger on, an electrical pulse is sent to the time delay relay which closes, passing electrical current to the defogger grid. Within the relay is a timing circuit which, after a finite period of time, is supposed to release, and open the relay switch contacts, turning off the current to the defogger. From your question, I suspect that your defogger relay has FAILED IN THE ON POSITION. Usually, this failure is fatal to the relay, which must be replaced. I have a Buick Century wagon 1996, and I ran into this problem. Have you noticed if the defogger switch lights up when you hit the brake? If so, you have a bad connection somewhere. The center brake light is on the same circuit as the defogger, so you may notice that the center brake light does not light up when you hit the brake. So if this is what's happenning to you, you either have a burned out center brake light, or the defogger terminals in the rear window aren't making a good clean connection. They are located on either side of the window near the top. What I did to fix this was to solder a small piece of metal to the terminal that was not connecting and it was thick enough to make the connection. So the center brake light works, and the defogger now works exactly as it should. Hope this helps.