I suggest checking that the fuse only blows when particular movements are attempted.
Another possibility is that the connection that is common to 8 motors has an earth fault. This would cause the fuse to blow when one direction only, of movement for all 8 motors. Earth faults can occur when the door loom rubs against the body, and movement occurs when the door is opened or closed.
To save fuses when checking, I would be tempted to connect a 21W bulb in place of the fuse. However that is easier said than done.