AUX
AUX is an auxiliary audio input, usually a small jack socket, that lets an external device play through a car radio or stereo.
AUX is the common name for an auxiliary input: a small socket, often a 3.5 millimetre jack, on a car radio or stereo that accepts sound from another device. A cable runs from the adapter or phone to the socket.
For retrofitting DAB+ to an old car it is the connection that gives the best result. In the 2024 TCS test, the sound through an AUX cable was clearly better than the sound from an FM transmitter. SRF also names the AUX input as the route for upgrading older FM car radios with an adapter.
Not every car radio has one. Check the front panel or the manual before buying an adapter.