On some of the 740's (and possibly 940's?) the same fuse that feeds the window motors also feeds the A/C compressor clutch. As the compressor clutch windings age it starts to draw more current to the point of blowing the fuse when you call for A/C or defroster. Consequently, when you turn on the A/C and it blows the fuse, you can't lower any windows for a bit of fresh air - idiotic design!
Try unplugging the A/C compressor and see if the fuse stops blowing. (Or refrain from using A/C or windshield defroster.)
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