Universal Car Remote Amplifier Subwoofer Equalizer Crossover Bass Controller:

Features:
Great for a sub amp volume control.
Connect the RCA from the output of this controller to the amplifier, and wire the car stereo output to the RCA input of this unit.
Adds a Dash Mounted Sub-Woofer Volume Control
4 gold plated RCA's (2 RCA input & 2 RCA output)
Level control knob
Frequency 150Hz - 40Hz
Gain 0db to -12db