Speedo working intermitantly

[timshap]

My speedo seems to come on and off..I think the sensor is fitted somwhere near the diff. Having waxoyled the surrounding area do you think I could have blocked the sensor?
The dials are 1994 black rimmed and are either stewart warner or etb
Does any one know have to find the unit or check its working? Its a borg warner box with a GKN diff.
From cold it appears to sit at around 10mph and later springs into life for a few minutes then back to 10 etc..
If I need to renew I am not sure which one it is...I think I can get one from clever trevor though..

Thanks