The speedometer drive housing is bolted onto the offside of the diff casing. The large drive log locates into the output shaft on the diff and drives the control pinion which locates into the bottom of the sensor.
The sensor and control pinion can be inspected and replaced with minimal fuss, only requiring the gearbox to be drained. There is a single bolt that holds the sensor into the housing and the control pinion should come out with it.
You need to remove the driveshaft and speedometer housing to access the drive cog though.
I can't for the life of me find it.. The speedo has stopped working similar to what this guy had:
http://www.306gti6.com/forum/showthread.php?id=132820&page=last
might have to just take it to a garage and get bent over?