TJB444

I have a 1999 t reg Synergie 160k on the clock. It runs well but have a particular recurring problem. Every now and then the speedmeter shows the wrong speed. It displays 20mph on idle and roughly 50 at 30, 70 at 50. It will then go back to normal after a while. This problem seems to occur after heavy rain or in damp conditions. I understand that if I were to clean the stepper motor electrical connection this might solve the problem. I would like to locate the stepper motor connection and give it a clean/dry it out. Pease could someone tell me where to find it ...or does anyone have any other thoughts what the problem may be... Thanks