Nick I think you are confusing Marc for me, lol. I found that the problem was in the actual odometer, not at the end connected to the wheel and not any problem with housing or cable. The part where the cable enters the odometer was stuck and would not spin.
I got into the odometer (so cool!) and put some WD-40 into the problem part and after lots of wiggling I have it moving freely again! The hardest part was opening the odometer...
Marc, I am still sort of confused about the magnet magic as well... but from what I understand, the magnet spins; it's magnetic field makes a disk connected to the needle start to spin as well. The disc has a spring/coil attached though. That spring fights back against the rotation so the disk doesn't just spin in circles... It only spins as far as the balance between the force of the spinning magnet and the spring pushing the other direction.
Hopefully there is a better explanation somewhere else but this is what I read... https://www.howacarworks.com/accessories/how-a-speedo-works