Crossfader/Volume control

Im not actually using a head unit.. i've got a multichannel amp wired into my car's speaker system with a portable mp3 player as the input. what i'd like to be able to do is hardwire a crossfader and volume control into my dash, that is completely independant of my mp3 player (or whatever input i plug in). I was thinking about picking up one of those light dimmer units at home depot for the volume control, but since that only has one input/output i'll have to do some re-wiring of the circuitry so my sound doesnt come out mono. but as far as the fader goes- any ideas? im trying to keep this project fairly cheap, so any DIY type faders would be preferred, but if you know where to buy a cheap one i'm all ears :)

A dimmer will NOT work in this instance... While they are SIMILAR to a passive volume control (which is what you are asking about) they are a wire-wound device, with FAR too much inductance for use with audio signals. Go to Radio Shack (I know everybody is going to laugh) and get a pair of these. Stereophile magazine loves them, gives them a Class B rating in audio quality, which, when you consider the OTHER gear that lives in that category, is VERY high praise, indeed! Laser trimmed, and of decent accuracy from unit to unit, they are an EXCELLENT value at 3 bucks each... You need a pair, because you will have one for right, and one for left channels, and this will give you your balance as well... If you decide to NOT take my advice, whatever you do, do NOT buy a wirewound rheostat or potentiometer... your mp3's will sound worse than they already do... While not as good a potentiometer, getting two of these will allow you to use one for the front channels, and one for the back channels, giving you your fader capabilities, at the expense of balance control. I do not know how important having a fader is to your application, I am just throwing out options for you...It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."