got a rev. light switch from a scrapyard vw [polo i think] for my T25 which worked, but make sure your old spacer /washer is replaced on the 'new' one or it may stay on all the time!
clean up connections first and check you've got a supply to the switch. A 12V bulb with one wire on the switch terminal and the other to a good piece of body metal should light it up. If you have a supply, put vehicle in reverse with ignition on and see if the bulb lights when applied from the other terminal to good body metal. If no light; switch not working, if light goes on problem lies between switch and reversing lights.
Additional note from 1.9 DG gearbox recon experience: the switch operating mechanism can be put in back to front
1985 1900DG wc liesuredrive hitop with small amounts of rust