So imagine my surprise when mine failed. I've been massively careful about oil, and it's never seen a drop in my possession. OK it could have happened before I bought it, but it looked spotless and had been with me for three years and a fair few miles without a problem. This weekend I pulled the cover off to inspect the brushes, and they were clean as a whistle, and plenty of life. So I did a couple of other checks, and found the rotor is very high resistance. It should be a few ohms, but was actually tens of thousands of ohms.
I haven't pulled it apart any further to see why, but I suspect it's location at the front of the engine halfway up has allowed salt water in that has corroded the windings. It did happen just a few minutes after the car had been washed, so maybe that finished it off?
I'll have a deeper look and report back!