Other than one weekend out of the year during South Haven's Harborfest (which took place June 16-18) you don't get the chance to go inside the South Haven lighthouse. Well, not anymore. I am helping you out by providing a link to a short (just under 1.5 minutes long) video showing the inside of the lighthouse along with some history of the lighthouse provided by Rich Shadduck from Harborfest. My wife and I toured the lighthouse a few years ago during Harborfest. The interior was rather boring as there isn't anything in there but the stairway and the red light at the top. However, the view at the top was great! It was kind of hazy the day we were there, but you could still see quite a way in all directions.
Watch the video, courtesy of WWMT Channel 3, here.