i may be wrong but this is how i see it - the mtx75 uses the speed sensor for the speedo as when i disconnect mine the speedo doesnt work - im then guessing the gearbox you used originally also had a speed sensor? if so just match the connector for the speed sensor - you may and probably have to look at wiring diagrams as they will be different - hopefully the fiesta , if it has a speed sensor originally , will also be 3 pin...that all may be wrong ...hopefully someone can shed light?