[shorod]

Are you absolutely sure the noise is coming from back there, and it's not just a sound reflecting around to make it sound like it's coming from back there?

Does it still make the sound if the car is not moving, but the A/C is on? What about is you turn the blower speed down? With the car not moving and the noise present, climb into the back seat and try to pinpoint the sound. Open the trunk and listen in there.

There is nothing that I can think of specifically located behind the back seat that would be associated with the A/C or wipers. However, there are relays located in the spare tire well with the battery that could be buzzing....

I'm suspecting from your description though that you have something in the blower motor squirrel cage (like a napkin or piece of paper) and it just sounds like it's coming from the rear deck.

-Rod