Nikon D300S viewfinder dust removal

During my landscape photo shoot this past weekend, I noticed some dust when looking through the viewfinder. I completed the following:

1) Clean the image sensor with the camera menu
2) Clean the lens
3) Switch lenses
4) Clean the outside of the viewfinder

In each case above, the dust still exists

5) Photograph a blank, white wall, analyze the image and look for the dust. The dust doesn't appear on the image.

This leads me to believe that the dust exists on the inside of the viewfinder only, and, fortunately, it will have no impact on the shot. Is my thinking correct? If I am correct, how do I clean the inside of the viewfinder?