When you refer to "replacement headlights," I'm assuming that you are talking about the entire assembly, correct? ...not just the bulb, correct?
From what I've been told, if you are purchasing and replacing the entire assembly, it has to be trimmed slightly in order to fit. Even Alfa had to do this with their initial installation when the coach was being built. This is my understanding.
Examine your old one and note the difference in shape, etc. Then take a Dremel and you should be able to trim the new assembly in the appropriate place(s) so it will fit. Again, examine the original and trim the new one so it resembles the old one.
I'm not sure exactly how much or where you have to trim but maybe somebody that has a SeeYa that has done it will chime in to this thread.