Hi to Low converter (adjustable). LOC2SL.

Hi to Low converter with Add-On adapter and remote control knob. Black.
LOC2SL. 

Amplifier add-on adapter allows you to connect a receiver with no preamp outputs to an amplifier without speaker-level inputs.
Converts car stereo speaker wire outputs to RCA cable outputs. Allows the addition of 2-channel or Mono amplifiers to factory to aftermarket stereos.
Includes mountable remote controller of adjusting subwoofer lever from the dash. 
Provides amplifier remote turn on output without removing car stereo. Works with high power car stereos up to 80 Watts per channel.
Easy to follow installation instructions.