I have 1991 RS 305. When i press the rear defrost switch the light comes on but it doesn't defrost the window...Any and all suggestions, how do I tell if things are bad..thank you.

Check the electrical connections at the hatch struts. The passenger side connector on my '92 keeps working itself loose.

Okay, I checked them both and they are both fine, any other suggestions? Is there a relay or fuse for it?

Look in your fuse panel and you will see two silver colored rectangular fuses/circuit breakers. One of them controls the defrost grid IIRC.

If all else fails, I think I remember someone saying he found a fault (in the electronics/relay) in the switch itself.

When i turn mine on sometimes it dosent come on but sometimes i can hold it up and the light will come on and then the light will imediately go out. When there is power applied to the rear window of the car, the defrost window. It looks like two lines toward the center of the grid are working the rest are still frosted. Kind of weird becaust it looks like the connections are at the top and run down the glass. so it looks like if there was a break it would work from the top down to mid point and no further. I dont know much about them but i do like how it helps.

Well, you certainly have breaks in the heater strips. They could have worn out by themselves but they are also very vulnerable to physical abuse. You'll usually find the connections on heavy vertical strips going down the sides of the window. These ensures that you get full voltage across the thin horizontal strips so that they will be the ones generating heat.. You'll have to buy a (silver paint) repair kit for heater windows, acetone for cleaning, tape for masking, and maybe a magnifying glass and/or a voltmeter to locate the breaks in the strips if they are hard to see. (There will be full voltage across each break.) Repair the window first, maybe the switch will work properly with the full load from the window restored.

I don't know how well this stuff works but check out this site:
http://www.frostfighter.com/

They have everything from a repair kit to whole new defroster.

I had a similar problem- the light would stay on a while without defrosting anything then shut off. If I hit the switch to turn it on again, it would buzz. Strange. Then it started working right, all by itself.

i figured out a big part of it but im not getting but two of the strips acrossed the center section of the window, the fuse panel is mounted upside down and there is a circuit breaker in the fuse panel for the rear glass, i removed it and barely stuck it into the holder just enough to make contact and touched both points with a test light, power on one side none on the other, removed the circuit breaker completely and congratulations i have a new baby rattle for a circuit breaker, installed new circuit breaker, bam the switch works now ive just got to take time to get the whole glass working.

p.s. there just isnt enough time in a day!!!!!!!!!!!!!

which circuit breaker is the one for the rear defrost? i looked through all my fuses, can't seem to find it. I know its one large silver one, there are two... Which one is it? THANKS!!!