problem
Fuse 9 (8 A), lid-card A (Radio) and C (Soundsystem), trunk battery, Bose amp, fiber ring on late cars. In that order. Not fuse B — B is the A/C clutch.
Engine-bay fuse 9 (switched, 8 A white) then lid-card letter A (Radio) and letter C (Soundsystem / CD / door lamps). Slot B is compressor / kickdown, not audio. Then voltage at the radio plug. Accessory dropped at the ignition switch will also take the cigar lighter with it.
Bose amp in the trunk, fiber-optic loop on Series III, antenna amplifier in the rear fender. Aftermarket head units that ignored the fiber loop are a staple of ‘it died after the new stereo’.
Constant 12 V missing. Battery terminals in the trunk, or the constant pin at the radio. Not the switched fuse.