Integrated HD Radio now common in U.S. models
Nowadays all the newer main brand car stereo models featuring HD Radio have it integrated rather than via an external device.
My first automobile HD Radio receiver was an external Alpine tuner box that plugged into the back of my Alpine AM/FM/CD unit with a proprietary plug and socket. The antenna plugged into it instead of the main unit. It worked well on road trips through the U.S., which was where & why I bought it just as HD Radio was first rolling out.
3 years ago I replaced the Alpine with a Kenwood ordered from the U.S. that also uses an external HD Radio tuner module with its own proprietary plug and socket and that the antenna is plugged into. Once I installed it the originial tuner option on the main unit disappeared and all tuning functions were taken over by the external unit, all controlled by the main unit's display and buttons. Reception improved on both analogue and digital stations as the quality of the tuner was markedly superior. Kenwood doesn't seem to market those external units anymore, now that HD Radio tuning has been integrated into most U.S. car stereos.