Doesn't sound like there is an easy fix for that. Our Family Truckster is a 2003 Lincoln Town Car, and the climate control in there is a pretty sophisticated gizmo. Not exactly a bolt-in aftermarket accessory type of thing.
If you have a coupe and keep the windows shut, the Mustang factory air should keep the interior at a fairly constant temp. The heater and AC are pretty potent. I have an '05 'vert and I have found that over the course of a year that it heats up pretty quick in the winter and it cools down pretty quickly in the summer (and temps during our summers here are like 105-110 in the shade).