Fiesta road speed sensor

[M-Ster]

Hi

I have been informed by my mechanic that the road speed sensor appears to have gone on my 1991 xr2i 1.6 8v

Does anyone know how to change it?? i can pull the speedo cable straight out but where do i go from there do i just take it off the bottom of the speedo and do i have to take the other plastic bit off the gearbox??