With the power to the rear defroster on, check both connections to a ground in the car. One should show about 12V, the other should show about 0V. If both show 0V, then your supply is broken somewhere. If you get 12V and 0V, but it doesn't work, then you've got a bad ground, or all the window elements are broken.
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1995 Buick Lesabre Custom 73000 miles
2002 Pontiac Sunfire SE 103000 miles
1997 Pontiac Bonneville SE 91106 miles Totaled by attempt to steal the car!!
R.I.P. 1978 Ford Fairmont 83000 miles

FTN