If you want an inline volume control (PAC LC-1)but like doing stuff yourself, you can make one for cheap. Obtain a dual 100k linear taper potentiometer and two 15kohm .25 watt resistors. You will also need two RCA female connectors or pigtails, I buy all this stuff from www.allelectronics.com
. The splitters I hacked up were 1$ ea. The 15k resistors paralleled with the wiper to ground make the 100k pot into a 12k analogue taper pot. What that means is that when the pot is at 1/2 turn, it will be roughly 1/2 the volume. With a linear taper pot, it would be 1/2 the resistance but not 1/2 the volume. Log taper works better for volume controls, this method "sounds" better than currently available log taper pots. The PAC LC-1 is a simple dual linear pot without this useful modification.
Solder the wires as per this crude drawing. It is drawn looking at the back of the dual pot, shaft facing away from you. You don't need an enclosure, you can panel mount them in your dash or whatever. This control goes in line with your head unit output before your amp.