72 Dart gauge cluster electrical problem

There's a fusible link in the circut near the starter relay. There should be a yellow tag saying LINK. When there's a short this piece of wire is designed to melt and break contact. Think of it as a low-buck fuse. Look in the center of the diagram and you'll see it wired into the starter relay circut.

http://www.valiant.org/electrical-diagrams.html

Hope this helps!