Reverse lights/demister relay no-go

[Porkchop]

Slowly working through all the niggling faults on my 84' 240 estate and am yet to nail a couple of them and would appreciate any feedback I can get. We haven't had reverse lights for a very long time, long story short, we are on our third gearbox having destroyed a couple of them slogging trailer loads of fill up our winding gravel driveway. We no longer deal with the mechanic that did the job(s) so I can't ask him but each time we have had a gearbox swap the reverse lights remain non-working (despite having asked him to investigate). As far as I can see the wiring is going where it ought to go and I can't believe that the switch has been a dud on all three gearboxes.
I suddenly had an epiphany yesterday when I realised that perhaps the relay has developed a fault as on the fuse compartment listing No 12 fuse services both reverse lights AND rear demister (neither of which have worked for ages). After several fruitless hours spent trying to track down the relay location under the dash I used a test light on the demister at the tailgate loom and found I definitely have power, there are a couple of small breaks in the element grid that I hadn't spotted before that must be causing the issue. I still haven't had any joy on the reverse light situation and would greatly appreciate any input from forum members who have had similar issues....