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Drop a weighted string or sash chain down through the gap between the jamb and stud on the side of the doorway until it can be seen at the bottom of the trim piece.
Pull the chain back up until the cable is all the way through. Disconnect the cable and pull it taut.
You can now run the cable across the top of the door and feed it down through the void on the other side of the door. Replace the trim and molding.
Excerpted from Do It Yourself Home Improvement
© Dorling Kindersley Limited 2009