You could do a "high level input" instead of using RCAs. Most amps come with the option and if using the amp strictly for subs you would have to switch the amp to LPF(Low Pass Filter). Also an option available on most amps.
To use a high level input all you would have to do is run a set of wires to your rear speakers or to the speaker wires coming FROM your factory amp, if equipped. You'll need pos and neg from the left and right speakers/signals. Hook them up to the high level input on your amp. Run the power. Hook up the sub/subs. Enjoy the BUMP!!!!!