Neither of my heated mirrors were working recently. I also struggled to find which fuse fed them but I never dug too deep as I had the car going in for some warrenty work on the brakes and so I got them to check the mirrors out too. Both elements were open circuit and so they replaced both mirror lenses. The elements are famous for failing I think. You will probably find more info on here about them, including links to you tube clips on how to replace them yourself if you need to.
It's also useful so with one more more click to the left [i.e. to the nearside] the mirrors fold if you have to squeeze in tight in a lane or meet something with even bigger mirrors[!] at the same height on a narrow road.
My driver side heated mirror packed in also, brought it back to Agnews in Belfast where they replaced both passenger and driver side mirrors under warranty without a debate....Supposedly the heating element packed in, same as andy5mart above....!!!!!!