the sensor for the speedo ,now you need to look at the back of the gearbox , right round at the back and you will see the sensor coming out of the gearbox ,unless its a cable then there will be a cable instead, but if its a wire you need to check the connector , now one of the wires is a feed the other is a earth and the other is a signal wire , and iam sory i cant tell you which is which with out a wiring diagram. but if its not got a sensor on the gearbox the signal will come from the abs computer ,for me to have more of an idea i need as much info as possable because the early meganes suffered from poor earth connection on back of the engine on the left hand side the earth which goes from the engine to the bulk , check the earths br looking and pulling the lead to see if it has broken.
mark, April 2008