If NEITHER rear window will go down, I would suspect the cutoff switch that intentionally disables these windows so small children who stick their heads out the windows will not be harmed.
The current from both windows is routed thru this switch, which could be defective or somehow jammed in a closed state.
If so, I would think applying power to the leads inside the door will cause them to operate. But I'd check the disable switch first. On my cars, it has a small logo of a child on it.
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"