Most vehicles are fitted as standard with a FM/AM Aerial with a coax cable fed through to the rear of the stereo/radio, the connector on the end of this cable can vary & when changing to an aftermarket stereo/radio the connection on the aerial coax cable will most probably have to be changed.
On certain vehicles a male ISO connector can be found on the end of the aerial coax cable (as pictured)
This is exactly the connection required for most aftermarket stereo/radio's & no action is required to install your aftermarket unit.
On a lot of modern vehicles & German branded vehicles such as Volkswagen, BMW, Audi etc, are fitted with a FAKRA connection (as pictured)
When changing to an aftermarket stereo/radio, usually in most cases a Powered Antenna Booster (FAKRA to ISO) (as pictured) will be required to keep the same strength signal reception as before, also a male ISO connection will be required to fit between the booster & aftermarket stereo/radio in most cases.
Also certain models are fitted with a dual FAKRA connection, which is exactly the same as a standard FAKRA connection, but with two FAKRA adapters as one dual plug. One connection is for FM & the other for AM, on these models a duel Powered Antenna Booster (FAKRA to ISO) (as pictured) will be required when installing an aftermarket stereo/radio.
Other less common connections can be found on American branded vehicles such as Chrysler, Chevrolet etc
When changing to an aftermarket stereo/radio, a connector converting to ISO (as pictured) will be required.
When changing a Factory fitted stereo/radio to an Aftermarket Unit, basically you need to go from either one of the standard connections to this aftermarket ISO connector.