I just did this job and will never do it again. What made the job marginally easier was removing the vent assemblies from the car. Tire soap made seating the seals a bit less arduous. As for your problem, I didn't trim any part of the seal and the wings close relatively easily and seat properly. I think the more the wings are used the better they will operate. I guess it depends on the actual seal manufacturer whether they fit well or not. Mine required substantial fiddling to get them installed since they were a bit oversized.