I agree with Bill.
We don't use the wheelchair seating for Illuminations because you have to get there much earlier than if you just find a spot. It is usually quite busy and they park the wheelchairs nearly touching each other along the fence.
Even if you don't get there an hour early, you can still find good spots. Before it gets dark, look for the barges that are parked in the lake in the middle of World Showcase. If you have a clear view of the barges, you should have a good view of the whole show.
The wheelchair viewing areas are marked on the park maps. If you look as you go around WS during the day, you will see the areas. They have some permanent chained areas marking them off and there are signs that indicate that the spots are for handicapped access.