99s are 5"x4.5" pattern (basically Ford passenger car for many years)...the rim/wheel size is generally marked somewhere on the inside of aftermarket wheels...factory wheels are hit and miss for markings. You'll note on the wheel chart in the article that there are a lot of Mustang wheels that will fit, but the trick is getting them with the correct offset. Some folks use spacers to get the offset they want...with mixed results (I'm not a fan of spacers). Do you know what the wheels you are looking at came off of...that may give a clue to fit on your car?