Mine is a RHD 71 CS, and the reset is via a cable that runs out from the speedo and exits on the panel below the steering wheel (near the little buttons for window demist / hazard lights etc). There's a selection of things there on most CS's it seems - someone also fitted an electric aerial switch there as well years ago! One awful possibility for you is that the cable has been removed/fallen out and not been replaced... cheers, Graeme