well even if the speedos arent working theyre is still a problem with the motor/gearbox if there is deffinatly power going through it. Its possible that the screw put through the black plastic is done up too tight.
Just to check it, see if you can turn the output shaft by hand easily. If not then thats your problem. The screw will be pushing into a steel ring inside, this ring has lots of knotches in it and the screw wants to be screwed into one of them. Tighten up the screw untill there is a slight amount of play in the output shaft before it starts to turn the motor, atleast then you know it isnt too tight.