<P> Amplifiers increase the power level of audio signals . Some head units have built - in stereo amplifiers . Other car audio systems use a separate stand - alone amplifier . Every amplifier has a rated power level sometimes noted on the head unit with the built in amplifier, or on the label of a stand - alone unit . </P> <Ul> <Li> Car speakers and subwoofers . </Li> <Li> Damping: sound deadening material is often used in the door cavities and boot / trunk area to damp excess vibration of the panels in the car in response to loud subwoofer bass tones, especially the boot / trunk . </Li> </Ul> <Li> Car speakers and subwoofers . </Li> <Li> Damping: sound deadening material is often used in the door cavities and boot / trunk area to damp excess vibration of the panels in the car in response to loud subwoofer bass tones, especially the boot / trunk . </Li>

What year did they start putting radios in cars