Here's how to get mildew stains gone completely.
After you have spent $$$$ on a ton of products that dont work, consider this, it worked on my 1995 205. The upholstery shop told me i needed to replace it because the stain was permanently set in, so I had nothing to loose in experimenting with what could work. Fortunately I found something that did work and the interior looks great!
If your interior is not torn but appears to be permananetly mildew stained and nothing will get it out, I found the answer. Take a fast evaporating PAint Reducer (Ex: PPG DT870) and a box of clean rags. Poor the reducer on the rag and wipe a 3in by 3in spot on your seat and it will return to its original bright color. Be careful not to mix applying areas that are different in color. IT takes along time because you have to change out rags every time you wipe a spot. I used rolls of paper towels folded into squares. The paint reducer will temporarily soften the vinyl , but since its fast evaporating, it will do that for only a few seconds. Afterwards apply some vinvly protector over vinyl after done with the entire boat