If it is a factory 1200 tacho you will find on the wiring loom right behind where the tacho sits there will be 1 loop of coloured tape ( on mine it was blue tape ) undo this loop and you should see a Black wire with a white trace! this will have 2 terminals connecting this wire together! disconect this and on the back of the tacho there should be a wire of the same colour looped through a plastic loop, connect the other wires ( one to each end) of this wire and the tacho will work!

If anyone else has found the tacho wires taped under the dash differently pls feel free to say so as I am just going off what I have experianced on a few 1200's that I have connected the factory tacho too!