Your last message gave me the clue I needed (and the encouragement) and I used a spade bit (bad use of a not very accurate tool) and I pried up the three tabs that appeared to hold the top winding and the soft iron the other part is made of (it holds the magnetic fields needed for the motor to operate) and actually look directly. I reasoned that the motor was toast, and I had little to lose by playing around.
There is indeed a rotating flywheel underneath, which spins very freely with little resistance. I held the motor up by just the gear, and tried spinning this with a fingertip to see if the torque gets transfered through the gearbox.
The drive gear did turn, so the gears work and the trouble must lie with the part I removed.
The upper coil was held by six 'petals', three above and three below, that are folded down to be press-fit inside the aluminum shell of the rest of the housing. Three of these are just short ones that have holes for the tabs that are folded in from the cap to cover the insides, and the other three extend down further. They all extend around the rotor and have a fairly close tolerance, around 1/32".
This part had been compressed when the mount slid into the seat back of my van. I made sure these legs were straight and put the coil back.
The disc still didn't rotate freely, so I checked to see if there was still clearance around it. There was, so I pried it up a bit and this time, the disc would spin freely. I pulled the winding again, and all the legs had no protruding bits. I looked at the part and noticed that the legs were squashed down slightly so I used the drill bit again to raise them, and I made the legs of it right angles.
Now it would spin freely with the winding placed. The motor was binding on the top.
I connected it to the circuits of the mount and it works properly now. I don't know if it tracks at the right speed, but I'll find out tonight.
Thanks for all the help on this people. See you at Table Mountain.
PS. Now if I can just fix the van as easily.