I have a 98 Mercury Grand Marquis GS with 168000 miles. About a month ago, I turned on the a/c and discovered that the fan speed setting only worked in high mode. There are four fan speeds and the lower three didn't work. However as suddenly as it happened, a week later it corrected itself and worked normally. Now it's doing it again. Any idea on what might be causing this problem and what can be done to correct it? (It also does this when the heater or vent or defogger is turned on not just the a/c). How much is a ballpark cost to fix it? Also, is it a do-it-yourself job?