Now that the weather is cooling in the Northeast, my rear window has moisture or frost in the morning. I've found over the past week that when I have the lights, radio and front defroster fan on, and then push the button for the rear defroster, that it cycles off after 30 seconds or so.

If the wipers are on, it shuts off almost immediatley.

Perhaps an overload of the circut, or incorrect fuse?

Help!

The relay the rear defroster is on, relies on sufficient voltage to keep the contacts closed. I wouldn't doubt with all those accessories on, your voltage drops and opens the relay on your rear defrost. Try swapping it out with a new one and see what happens.