It also occurs to me that many cars come pre-wired for trunk-mounted CD changers. Such devices need audio, power, and communication lines with the dash. With the right adapter connections and knowledge of the wiring harness pinouts, one could even avoid the power and RCA runs to the trunk. The power line might not be up to the task of a high-wattage amp, but might be enough for a low power amp. Of course you'd only have L/R audio lines to the trunk and therefore wouldn't get fader control. But it's a thought anyway.