Electronic Speedo

Does anyone have any suggestions on how I can get an electronic speedo to work on an ultma 6 speed trans. I have tried 2 sensors to no avail. VDO says that the speedo will work with a stock sensor, but I can't get it to work. I looked down in the sensor hole in the trap door and can't see a magnet or anything on the gear to send a pulse to the sensor. Has anyone else had any problem with this? Any info would be greatly appreciated.