Adapter
An adapter adds DAB+ to a car radio or stereo that lacks it by receiving the signal and passing on the sound.
A DAB+ adapter is a small box that receives DAB+ and hands the sound to a device that cannot. It has its own antenna and connects to a car radio or stereo, often by an AUX cable or by Bluetooth.
TCS and Kassensturz tested adapters in 2024. TCS found that all four adapters it tested received DAB+ well, and that sound over an AUX cable was clearly better than over an FM transmitter. Kassensturz found that adapters for stereo systems failed with the antennas supplied.
In short, adapters work, but the antenna matters. The car guide has the details.