2003 Mustang adjustable illumination fuse keeps blowing

If this were my car I would pull the head light switch and inspect for evidence of over heating. If any is found replace the head light switch. These fail at a fairly high rate.

If none is found I would use a regular test light to see if there's a short to ground on the circuit. Pull the fuse. Put the test light pig tail on a known good battery positive. Confirm that the test light does light when touched to a known good ground. Does the test light light brightly with the fuse removed when touched to the "load" side of the fuse? If so, there's a short to ground in the circuit.