On two trackballs I found the ground wire was intermittent and eventually broke completely. When it was intermittent it stopped the pointer from moving, disabling the trackball. Finally the trackball was unusable.
Right after the exit of the wire from the body of the trackball was a bend in the wire. I cut into that carefully, cutting the PVC black wire, exposing the 4 wires and the broken ground wire, which also acts as a shield. I sliced the black plastic open. about 1.5 inches in length, and pulled the remaining ground wires from each side. I then twisted each end of the grounds wires, bent the USB wire so they could be fastened to each other, and soldered the broken, pieces.
Then, I cut the soldered ground wire to about half an inch. I covered the entire bent section with electrical tape tightly wound over the bend. I tucked that under the circuit board inside the mouse, and brought the remaining repaired USB wire out the end as it was before.
It takes a bit of finesse to put the wire under the board and run the rest of the wire out to the grommet to the end of the trackball front. I then put the two pieces, top and bottom together, and when it clicked into place, put the screws back in to secure the top to the bottom. Both trackballs were fixed and ready to run.
Note, when opened the trackball had lots of finger dirt accumulated inside. I took a stiff toothbrush and removed as much as I could.