The 'Stereo Out' socket is too quiet or silent
The ‘Stereo Out’ socket is a 'variable' output controlled by the volume of the radio and isn't a fixed 'Line Output'. It is possible that when using the Stereo Out socket the radio was turned down - inadvertently providing little or no output. If the volume is turned down, it is not sending a line level output - only when set at around 3 quarters full volume is it the output getting close to line level output.
The way to use the 'Stereo Out' is to connect as normal (e.g. to your Aux In device) and turn the radio volume up to roughly 3 quarters and then mute the radio. An another option is to plug a 3.5mm jack plug into the radio’s headphone socket while using the Stereo Out socket, which automatically mutes the radio speaker. This method gives you control over the variable 'Stereo Out' socket and supplies a good enough signal to your stereo's Aux-In or Line In. On top of this you can now control the overall volume through your stereo system or your radio’s volume dial.