A guy that I know (use to work at a pretty high end stereo shop), said they never used those RCA converters and just tapped into the factory speaker wires and put RCA plugs on the other end... He told me most of those RCA converters where junk and would cause noise problems, etc. So they just ditched them and spliced directly to the amps. This gave more input power to the amp, and they would just turn the gain down a little to compensate for it (this would also reduce the possibility of noise issues)...
I have been in probably 10 cars like this and they sounded great with factroy head units, replaced speakers, and aftermarket amps. so it does work.
Also you might check with CrutchField.com they can usually set you up with something and all the connections to make it a plug and play operation.
Good luck,
Dave