When it comes to changing the radio in your vehicle, not all of them can be as easy as a simple straight swop. Some will need interfaces and adaptors for both radios and speakers. The Honda Civic for example, to change the factory radio you would need a facia panel, a harness adaptor and antenna adaptor to install any aftermarket radio.
Below is a before and after image.
With these items pictured below you are able to change your factory radio whilst keeping the dash undamaged and the electrics safe whilst keeping the use of your steering wheel controls. There are also interfaces available for a vehicle where you might have the factory parking sensors through the factory radio or even climate control information displayed. Interfaces and adaptors are also available, please get in contact and we will help you all the way.
As for the speakers a lot of factory fitted speakers have a pre moulded collar built as part of the speaker that bring the speaker away from the door which may allow clearance for things like the windows and there mechanisms.