The radio dial-light lead (the non-fused lead) connects to the instrument light lead on the light switch (Instrument & Ignition Lock Light lead).
light switch connectors The installation instructions for the 47-55 radios suggest connecting the fused leads to the ammeter:
1954/55 radio power ; and,
1951 (1947-53) radio power The advantage/disadvantage of connecting to the ignition switch would to allow you to choose a lead that is always hot or a lead that the hot only when the switch is on.
