Last weekend my daughter wanted to be able to play her portable player trough the car stereo, and after dismantling it, I found the volume control is digital and was unable to tap an input at the volume control.
By looking at the layout of the PC board, decided to insert a stereo 3mm canceling type jack (5 pin) at the volume control chip inputs, and mounted it on the front panel.
Did not find such type of jack in my parts bins, and canibalized one out of a old PC sound card. Works well, simple, and does not need relays, transformers or sensing circuits.
When the plug is removed, the radio plays its source normally. When inserting the plug, only the portable gadget feeds its audio source. It sounds much better than the FM modulator we tried before.
Perhaps it will work for you.
Miguel