Car is just coming up for three years old. Not under warranty as one service was missed. Car failed it's test Mot. It's first test. Rear fog light in the tailgate not working. All other functions in that tailgate cluster working. Light on dash shows on. Checked fuses. Swapped relay around in fuse box, same spec obviously. All working. Pulled off connection in tailgate. Sprayed with contact cleaner. No corrosion no damp or condensation all as new.. This is an Led cluster. Mazda want about ?180 to replace and ?125 for investigation. Plus pay for another MOT test ?50. Outrageous really, bearing in mind a few years ago a light bulb would cost about ?4 and DIY. Any ideas?

 

There was a recall for rear lift gate module on certain years.
Not saying that is your issue but it is a possibility.
My 2020 was one of them though I had no issues they did replace it and I've had no issues since.
On my 2020 the module does attach to the rear wiring harness which includes all the lights.