SCOSCHE SLC4 Car Stereo Speaker 4-Channel Audio Lineout Converter/OEM Amplifier Adapter with Adjustable Level Controls
 
The Scosche SLC4 4-channel adapter line out converter for car stereo audio allows you to connect a receiver with no pre-amp outputs to an amplifier without speaker-level inputs.
 
The SLC4 handles a speaker-level signal up to 80 watts RMS per channel, then converts it to a line-level signal you can plug into your amp's input.
 
This speaker wire to RCA speakers converter is compatible with high power car stereos up to 80 watts per channel. Perfect for fast installation when adding a subwoofer to your factory car stereo system for a new sound experience.

This converter also allows some flexibility when installing new headunits matched to factory amplifiers. This will allow fine adjustment of each channels signal level so you can make sure your aftermarket headunit is not overdriving your factory amplifier with signal.