You really should take the signal from the rear right AND left speakers using a summing circuit, otherwise there may be parts of the song your sub will miss due to the fact they are being sent to the right channel and not the left. See here for how to build said circuit, should cost you 50c in parts from Jaycar.
http://www.termpro.com/asp/pubs.asp?ID=115#note, replace 'center speaker' in this article with 'subwoofer' and keep in mind the input impedance of your amp is probably up around 250kohm when doing your calculations, and also that your amp has gain and crossover control, so it should be unnecessary to use that.
Of course you could just use one of these from jaycar and wire it in parellel with your rear speakers and then just RCA to your amp from there...
http://www.jaycar.com.au/productView.as ... rm=KEYWORDThe choice is yours

P.S. good call for asking first, I hate autobarn installers with a passion cause most of em are useless, there are guys on this forum who have made the mistakes and are more than happy to let others learn from their mistakes, giving you the best bang for your buck.