I have a 90. My book indicates FX3 was std for 90 onward, but I think this is a missprint.
Your correct the switch is located in between the seat switches on centre consol ("gtc" put up a good pic last week.).
There should be a module secured to the top of each Shock, basically a motor which drives a shaft installed thru the centre of the shock, when turned it changes the "effective" valving inside the shock from soft, medium to hard. This is set as required and does not change untill YOU change it. Not sure where control box is located. My081 posted last week that our friend Kiwi Col has this set up and sent his shocks away to the US, to be rebuilt, some $100s of dollars each. So maybe it was removed. Check around the car for stray connectors, easiest under the hood, or cut off and loose wiring etc.
It is possible the wiring was removed professionaly at conversion, more than likely just butchered and stuffed back into the loom tubing covers.
Maybe shocks were stuffed after 8yrs of service, and removal was cheapest option, especially if the converter was the importer. Time is Money.
But these C4's are really like driving a rocket sled, fantastic shape, no silly plastic headlights, mines as quiet as a mouse.
P.S Col my be coming to the Xmas luncheon c/w vette. Contact Club Sec.
Regards.