Ford Focus 9 inch Android Head Unit with Wireless CarPlay, Android Auto and 4G

Your iPhone connects on its own when you start the car, so there's no cable to plug in. It needs an iPhone with wireless CarPlay support; check yours in Settings > General > CarPlay. A cable still works when you'd rather charge on the way.

Maps, calls, messages and music from your Android phone appear on the car screen, with Google Assistant for voice control. It needs a phone with Google services, which most Android phones have. To connect without a cable, the phone must support wireless Android Auto too.

A SIM card in the head unit gives it its own internet connection for maps, traffic and streaming, so it doesn't need your phone's hotspot. You supply the SIM and the data plan; a data-only SIM is usually the cheapest way to do it.

The sound processor lets you tune the sound to your car: an equaliser, a delay for each speaker so the sound centres on your seat, and crossovers that keep deep bass away from small speakers. Start from a preset and change one setting at a time.

The built-in amplifier drives your car's speakers directly, which is all most cars need. If your car has a factory amplifier, as many premium sound systems do, it usually stays in use and may need an adapter, so check before ordering.

Connect a reversing camera and its picture appears on the screen when you select reverse. Automatic switching needs the reverse signal wired in. If your car already has a factory camera, check whether you need an adapter to keep it.

Mirroring shows your phone's whole screen on the car screen, unlike CarPlay and Android Auto, which show apps designed for driving. It usually needs a companion app on the phone, and paid video streaming apps often block it.

Digital radio gives clearer sound and more stations than FM where there's DAB coverage. It needs a DAB aerial: a stick-on windscreen aerial is the usual choice, or a splitter if your car already has a combined FM and DAB aerial.