2003 GT Convertible. Should be a simple fix, but my fan only blows on high. Would this be a switch or the fan itself. I am a fabulous plumber but the electrical is not my strong suite.
Typically this is a symptom of your blower motor resistor module needing replacement (not the switch or fan itself). Basically, the resistor network is a voltage divider that makes the fan blow more slowly. Since on high speed the resistor is out of the circuit (full voltage on the fan), high speed still works. Haven't had this fail in my Mustang yet, but replaced them in a few Ford Ranger / Mazda B-series trucks due to the same symptom. Search YouTube for "Blower Motor Resistor Mustang 2003" and you'll find some videos, but it is pretty simple with two hex screws, just up under the dash on the passenger side, forward of the glove box.
Rock Auto has this listed for a 2003 GT as a Motorcraft YH1699 (4L3Z19A706AA). The Motorcraft is about $26 or you can go for a cheaper brand for $8-$10. Can probably pickup at your local parts store or dealership as well.