To be more precise, look for a panel typically located below the steering column, near the driver's left knee. You might need to remove a small access panel or use a flashlight to see it clearly. Sometimes a small pull or release lever is involved to open the panel.
Keep in mind that some modifications may have altered the original location. If you can't find it in the typical spot, consult a 1972 Mustang shop manual or a wiring diagram specific to your model for more precise location information.