I just replaced this on my 1.9 JTS (M32 gearbox), the switch is fairly easy to access from above. Its on the front face of the gear box, pointing upwards and towards the radiator, and just above an "L" (ish) shaped bend the the radiator hose. I found it difficult to determine what size socket I needed to open it as it needs a deep-socket. It turned out to be a 22mm and the generic replacement I got is a 19mm.
To verify its the switch that's at fault, after checking bulb, fuse etc., I shorted out the terminals on the switch plug with a 7.5 amp fuse, then pop in the key, no need to start, and stick it in reverse - light should be on. Just be sure you have located the correct switch before trying this!