On 87-93 Mustangs, may be the same on earlier model cars.
The glove compartment has a big yellow button under the LH side of the glove compartment door. Press the button with the ignition switch in Accessory or Run and you should hear a loud cluck that is the solenoid unlocking the lock.
OR
If there is no lock cylinder in place, a flat blade screwdriver can be inserted into the lock cylinder hole and into the locking mechanism. Twist the screwdriver and the locking mechanism should release.