I haven't looked for it really hard, yet, but on the S3 it's right by the clutch MC.
I should have been more specific - the S3 in question is an '87, so it's the pulse generator. You're correct, though, with regards to the earlier S3's with the twin-pod dash configuration and mechanical speedo and tach.On an S3 it's a direct cable drive speedo that originates at the left side of the transmission tailshaft.
What you're seeing by the clutch MS is the tripometer that lights up the 'exh sensor' light on the console every 30k miles. (cable from trans to tripometer, then cable again from tripometer to speedo head. The tripometer can by totally bypassed by using an S2 speedo cable)