dodgy rev counter 300td

[jgs600]

when i bought the car i was told the rev counter didnt work, but ive noticed in the last week every now and then the rev counter will suddenly start to work, some times showing the correct revs and sometimes suddenly dropping back to zero while im driving. i probably need to replace it but was wondering if it might be a dodgy connection somewhere. does anyone know where the rev counter gets its feed from.
cheers

1990 300td diesel

 

[Rob77]

Good luck with finding the fix for this as I've had no succes so far