Rear window defroster only works sometimes

When it doesn't work, is the button on and illuminated, but the window itself isn't defrosting?
 
Have you unplugged it at the rear and visially inspected the plug and the contacts? May just need to be unplugged, cleaned, and some di electric greases added to it. It is right next to the rear spray nozzle hose which can dry out, crack, and leak.
 
I will check to see if the contacts are good...I was not aware there is a rear sprayer...hmmm...will have to look for that as well...on the 97 model?