Sounds like your in the US, isnt the c4 a classic auto box, if they were made with a vss they must have a long heritage.
Some schems on the net just show a shunt resistor across the vss sensor, looks as though they just load down the sensor to a low voltage, maybe you could try something along these lines using a battery drill to turn the sensor.
My car vss goes direct to the speedo, maybe theres some in the US that do the same, the i/p circuit from one of these might be a good donor, looking up the datasheet for the chip would be usefull if you went this route.