Mine's been doing it for the past 9 months, sometimes it doesn't work, other times it does but only lukewarm, but seems to get really hot with the heated windscreen on. It went in for a service back in Nov 2019, the mechanic tried it and of course couldn't find a fault, but did say it's probably an internal problem with the heater matrix(if that makes sense), it's a shame they only told me about it when I came to pick the car up as otherwise I would have made them change it.
I've had a look myself and I can see any loose connections, however I believe it works directly off the battery rather than from the alternator, so therefore I was wondering if maybe the electrolyte in the battery might be an issue? Does anyone know if it's a sealed or vented battery. I.e one you can top up or just have to replace.