I just measured the temperature and fan speed on my two 91 Reattas. The first one is a coupe with 78,000 miles, the second in a convertible with 13,000 miles.
Each car test was with all 4 vents open, the second test is with only the drivers inside vent open. This will be the one with the higher fan speed. On the convertible with 13,000 miles. All the tests were run with temp set for 60 Degrees, and fan speed on High.
Since the coupe has a fan speed of about 2 MPH lower than the low mileage convertible, I am willing to bet my evaporator core is dirty.