Aftermarket radio keeps cutting out with the key on but engine off But it works fine with the engine running why?

The issue with your aftermarket radio cutting out with the key on but engine off is likely due to insufficient power supply. When the engine is off, the radio may not be receiving enough voltage from the battery to operate properly, especially if other accessories are also drawing power. Additionally, some radiOS are designed to turn off to prevent battery drain when the engine isn’t running. When the engine is on, the alternator provides a steady supply of electricity, allowing the radio to function normally.