When we were sailing in the Sea of Cortez with Dina and Malcolm on Good as Gold, they had a new radar mast fabricated. The welders did a good job, and had led wire messenger lines throughout the mast to enable them to pull through the wires. Unfortunately the wires, hardened by the welding next to them, broke when they tried to use them. As each lead required multiple 90 degree bends, it was not possible to push a new wire or messenger through the mast. It looked like they would need to have the mast taken apart so new messengers could be put in place.
The solution was to use a pair of small, powerful magnets. One was threaded with Sailmaker’s thread and dropped into the mast at the top, and the other was used to pull it down the mast, with the magnetism reaching right through the wall of the non-magnetic stainless tube. This allowed them to pull the magnet inside the tube down to the exit, trailing the thread behind it. The thread was then tied to a stronger messenger line, and that line was used to pull the antenna wires down to deck level. These magnets are available for a few bucks at Lee Valley Tools.
This technique can be used on any wiring conduit you have exterior access to, but can’t push a snake through, so long as there is enough space for the magnet (and of course the wire). A larger, more powerful magnet used on the outside, makes the job easier.