If just one harness plug to radio, the radio may have a remote amp. The radio head/radio unit (installs in the dash) is wired to the remote amp (installs under seat or other area).
The amp receives the signal from the head unit for radio, tape deck, CD player, etc. The amp then amplifies this signal and powers each speaker.
If you are trying to installed another radio, you need to address the issue of the remote amp. If the replacement stereo has a built-in amp, you need to bypass the remote amp.
This by wiring the speaker outputs from the replacement stereo directly to the speaker wires that come from the amp.