Hi mate, I can tell you that I have just had this happen at worksop last month. Same as you, I accidentally left the sensor wire out of the motor, switched and it smoked then dead as a dodo.
Unfortunately, changing the sensor wire does not work. what happens is the speedo 'looks' for the orientation of the motor, does not find it, and goes mental. Its not even a repair jobby, if you have the receipt though, I got a replacement unit, (just don't say you left the sensor wire out), Kevan