Dim cluster

First thing. What model year Mustang are we dealing with? I'm going to assume 1999-2004.

There is a ground behind the center console that is shared by multiple devices including the cluster, GEM and radio. It is frequently messed up during a radio install. Recommend using a test light to "confirm" a good ground on all black wires and black wires with white stripes. Put the test light pig tail on battery positive. Probe the grounds to be tested. Each should light the test light brightly.

Measure the voltage at fuse F2.37. Verify that the voltage changes when the dimmer switch is rotated.

IF the voltage at the fuse does not reach full battery voltage, THEN suspect a problem at the head light switch. Perhaps consider replacing because the head light switch because over heating is a real thing.

If there's full voltage at fuse F2.37 then consider that more testing of the individual parts will be needed depending up WHICH part is dim. Likely more ground testing.

1999-2004 MY fuse panel schedule:

Lastly: Check out this thread on Cluster PC foil trace repair.