The car is a '96 S420 fitted with an Audio 10 cd player. I am trying to figure out how to get the VSS - speed sensitive volume control to work. The wire is present in the harness, I've tried voltage measurements and get some fluctuations from ~0V at a stop to ~6.8 - 6.9 at speed. I don't have an oscilloscope to measure frequency.

The radio has the feature. Is there something else needed to activate it?