Do you have to connect the RCA preamp output a car radio?
It depends on your specific car radio and setup. Some car radios have built-in amplifiers, in which case you would not need to connect an external RCA preamp output. However, if your car radio does not have a built-in amplifier, you will need to connect an external RCA preamp output in order to amplify the signal from the radio and send it to your speakers.