If we make an honest try for something that's unlikely, but at least vaguely possible:
*Ike sabotages the American advance. The Russians push deeper into Europe, making the Americans more afraid of them faster. They also take Finland.
*The Nazis manage to build workable atomics, and detonate one in the North Sea after first notifying the Allies so they can send observers. This proves both their ability to defend themselves and their willingness to negotiate, since they did not target the weapon at Allied forces or civilians, but instead provided a friendly demonstration.
*The government is taken over by some reasonably reasonable figure the Allies don't have to hate on sight. Speer comes to mind, or one of the Generals; any better ideas? Perhaps even Göring by a stretch?
*The Germans drop everything else and concentrate on fortifying Norway, at the same time preparing to negotiate.
If they play it really smart, they could then combine their impregnability and relative legitimacy with the threat of the Communists. That might possibly be enough for the Allies to leave them alone...
But they'll be viewed with hatred at home for deserting the Fatherland...