Corrosion - big time!

In an attempt to exchange the old, cracked windshield apron (Windlauf) I had to remove the 2 nuts which affix the windshield wiper mechanism. Unfortunately, this is made of Aluminium die cast and the nuts are zinc- plated steel. In 30 years those parts corroded into 1 solid chunk of white corrosion. Both threads sheared off as soon as I tried to loosen the nuts:

Here you can see the bronce bushings and part of the threaded dome + the nuts...
Now I have to find a new wiper mechanism..