Hi Dave. The moisture likely gets in from road spray while towing in wet weather conditions.
Removing the motor was a bit of a problem for me as the four small mounting bolts had some corrosion on them. I had to drill out two of them. Once removed there is a rubber casing over the motor housing that I was able to slide off without damaging it. There was some adhesive holding it on. Remove the two end bolts that hold the motor housing together. Then slide the housing off to expose the armature. The moisture was entering where the housing separates. This caused the small springs holding the commutator brushes to rust. One of the springs was dissolved.
When I refurbished the motor, I sealed it during re-assembly using silicone. You may have to replace the springs and brushes. A local electric motor repair shop had what I needed.
Dave