If a situation similar to the book Fatherland happened, what are the chances of some sort of revolution or coup of some sort? It's very likely that many would be tired of the oppressive regime after a while.
I thought up an alternate history movie or book about a British documentary crew going to Germany in the modern day, to do a documentary about the victorious Germany, which maybe is a little smaller than in some fiction. In Fatherland there is a European Union of German puppet states.
In the middle of the documentary, the crew gets caught in the middle of an uprising. Situations like that are happening all over, mostly in Eastern Europe, but also in regular old Germany. It eventually ends with a Soviet advance into Eastern Europe, and some sort of nuclear dirty bomb being set off in Berlin.
It's likely to happen in the case of a German victory.