I would think that as pre installation of air conditioning wiring and ducting was promised you have a good case to demand Interlaken install it retrospectively. The trick will be in getting them to do it!! I would have thought that as this is not something you could reasonably be expected to be able to pick up when snagging an apartment especially as there is no power available that they will be sympathetic. Please let us all know what response you get.
I am also a phase 2 appartment although not yet snagged so would be interested if you could advise what to look for to check it has been installed in ours.