I am unsure on your car but generally the rear window defroster stops working because of a separation of the two pieces of glass, damaging the internal wires. The only way to repair this is to replace the rear window. If you have 12v to the (i think) *driver's side* (on a w126) then it is the window NOT the wiring or relay.

-Joey

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1988 Toyota Camry Wagon......CRUSHED
1988 Porsche 944.......SOLD
1984 300SD.....SOLD
1995 Honda Civic DX... Totaled 11/13/08
1991 Madza Miata 193K and counting.....Mercedes t3 Turbocharger in the works 10 pounds per RWHP FTW