The two door switches connect between the GEM and ground. When either closes, the GEM outputs 12V to the dome lamp. You could have a short to chassis in the dome lamp assembly, the wire leading to it (less likely), or within the GEM itself.
Behind the passenger-side cowl panel, about half-way up, approximately at the level where the wires pass into the rubber conduit to the door, there is a 3 pin connector, gray in color. This connector carries all connections from the GEM to the dome/courtesy lights. If you pull apart that connector, and the fuse no longer blows, the problem is most likely in the light assembly. If the fuse still blows, my money is on the GEM.
GEM failures, however, are very rare. In 5 years on this and other Mustang boards, I have yet to see a reported GEM failure. If anyone has recently done any kind of work under the dash, I would immediately suspect damage to a wire before the GEM.