I don't know wiring colors, but if you want to clean up the signal, I'd look into a cheap signal processor to run RCA's instead of line inputs. Basically it's a powered block that you can tune what frequencies pass through to the amp. Works well with a factory head unit.
If not, maybe try the local dealer to see if they have wiring diagrams and just use the line level inputs. It'll be better than factory, but not as good as pre-amp outputs on an aftermarket HU.
^ It's available at Walmart for cheap and will help clean up the sound a lot.