Rear defroster

My rear defroster has stopped working? I've tested the wires in the window and they are fine. The volt meter says about 3 volts getting to the window, The on / off switch is working - the light comes on when I pushed the button. I'm asuming the voltage is to low to the window. What would lower the voltage and how do I find it??

According to my GM manual, it requires 12 volts, however it gives little information except on repairing the rear grid. Would have to start from rear and trace wire backwards, testing along the way to the switch(which may be bad). you could have a poor ground or a broken wire. I have no experiance with this myself, so am of little help. Might want to look into a Haynes manual for a wiring diagram perhaps.