Where in a 1991 Mustang 2.3L the Constant Control Relay Module is located?

The Constant Control Relay Module (CCM) isn't a standard component in a 1991 Ford Mustang 2.3L. That terminology is more associated with later model years and different systems. A 1991 Mustang of that engine size uses a simpler setup for controlling various electrical functions. What you might be looking for is related to the power distribution box, under-hood fuse box, or specific relays for individual components (like the fuel pump relay). To find what you need, you'll need to be more specific about what function you're trying to diagnose.