Depends on the Radio. Units like the Kenwood TK have a built in Amp to feed an external speaker . 13w amp from main unit and 5w from the remote head which is plenty for one like the center channel speaker. Units That have a headphone or line level output should not be used to drive speakers. A headphone jack on your phone for instance is not a real line level output but its also not designed to power a speaker. Its normally set for an impedance of 16 ohms or so and low wattage. If you plug in a 4 ohm speaker you could plow its micro amp. And line level is designed for extremely high impedance with almost no power, which feeds a clean signal to your amp.
If you don't have a built in amplified source I would just buy a cheap 12v mono amp of amazon/ebay they are $15 or less and will give you the power you need. Reality is you don't need more than 10w. I use the 5w on my Kenwood as its plenty loud so did not see the need for the 13w feed