The headlights on warning buzzer was not a stand alone option, but was part of the "Convenience Group" option for all 71-73 Mustangs and Cougars. It included trunk, glove compartment, map, under hood, under instrument panel (standard on verts), and park brake warning lamps. It also included the lights on warning buzzer, automatic seat back release (not available before 1/71), and glove box lock. Lot of nice options.
The headlights on relay is D1AZ-10C838-A (ID# D1AB-10C838-AA). The buzzer was a multi use item for the key, lights and belts warning. The buzzer went through several replacements so I'll list just a few and the engineering numbers to help you.
DOAZ-13150-D (ID# DOAB-13150-D or G)
DOAZ-13150-E (ID# DOAB-13150-B,E, or F)
D2AZ-13150-C (ID# D2AB-13150-A1A,A2A)
*D2AZ-13150-D (ID# D2AB-13150-BA)
*D4AZ-13150-B (ID# D4AB-13150-A1A, D5AB-AA, D5ZB-AA)
*The last two were the best since they had a radio suppression diode to correct a noise problem.
You can search by Ford part number on E-Bay, or contact Don @ Ohio Mustang Supply, Mike @ Motor City Mustang, or Midlife and see what they have. Good luck with your project.