"Head Unit" is the device in the dash that makes the sound in a car stereo system.

For instance, all of the following can be considered head units:

CD player.
Cassette player.
Minidisc player.
Mp3 Player.

They connect to other components, such as crossovers, amplifiers, and speakers, but they are at the "head" of the signal chain, hence the term "head unit".

RCA is the name of an american electronics company. The specific kind of audio cables that connect car stereos to amplifiers, and connect home stereo components to the main amplifier, are known as "RCA cables", named after the company that invented them. These are sometimes called "Line level connection cables" or "Patch cables".

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Tony Fabris