First disconnect your arm under the dash from the motor and try it.
It might just point out exactly where the problem is.
Also you should leave the wiper arms/blades off until the thing makes a pass and parks itself when shut off, THEN put the blades on.
Other than that the park assembly is probably bound up somehow inside the gear drive section.
OR, the motor got bound up on assy.
OR, it was a shitty rebuild.
Could even be all of the above but ya got to start somewhere.