Look closely at where the wiring harness feeds into the motor itself in the middle of the assembly. You may see some green or light blue crud there. If so, then you have the same problem I have run into on my first condensor fan and also a used unit that I bought from a salvage yard. The wires get soldered into the motor leads, and the solder joints "bloom" with age and water exp. You then get too much resistance at the connection.

I cut a small section of the bad wiring off the harness, soldered a new short piece of heavy wire in its place. I was able to lightly brush and clean the corrosion off the solder joint/wire of the motor itself, then reheat and add a bit more solder to the affected wire[lower wire] Fixed both condensor fans this way. Use heatshrink insulation on the new wire section to protect it.
I added silicone sealant to the outside of the solder joint on the motor itself to help prevent more corrosion. I wonder if the factory used acid type solder for this connection, as you don't often see this much corrosion on a rosin type solder joint. Hope this helps you out.