The answer is yes. You can connect speakers to the radio it self. The same type of speaker active part is on the amp as in the radio. The rear-deck speakers may have discrete(Transistor Amp versus a Integrated Power Amp) component amp.

The RCA adapters terminate the signal to audio transformer(the good ones). Then take the step-down secondary coils(for each channel) to a resistor network to RCA connectors. The Cheapy RCA converters just use resistors to step down the signal.

Martin