3 options:
1) Change the head unit to give you more than 1 pair of pre-outs. 3 is best, a front pair, a rear pair and a non-fading (for sub) pair. This is obviously the most expensive (but best solution)
2) Use an rca signal splitter. Very cheap - you'll do this for under a tenner. these essentially sit somewhere along your RCA cables (either at head unit end or amplifier end) and create two more RCA sockets/plugs (dependant which type you buy). Very littel signal loss. The only problem is, your fading will be a little messed up - the likelihood is that your current pre-out is a rear pre-out (most single pre-outs are) if you utilise this then you'll loose the ability to fade properly because everything will be driven from rear-based signals. The solution here is to swap your headunit-amplified cables over, so the front out puts of your head unit power your rear-fill speakers (if you have them) your fader will then work back to front, fading to the front will quieten amps and increase volume to rear fill speakers, moving to the back will quieten the rear fills and make amplified speakers (sub and front components) louder.
3) Use a line to rca converter. This little box of tricks will take an amplified signal (i.e. speakers wires) from your headunit and turn it into a pre-out. This means that you retain the fading abaility (as you'd use the front speakers outputs from your headunit). The downside is that, because this box of tricks is taking an amplified signal and converting it, the signal out the other end isn't as clean as a true pre-out signal.
You pays your money, you takes your choice!
