I don't mean to rain on the parade here, but if I'm reading this correct, it says connect an aftermarket radio as you normally would. This indicates to me that the Nissan Amplifier is actually taking it's signal from speaker level signals. Your mobo is outputting line level signals. There may be a buffer circuit of some sort inside the amplifier and an automatic gain circuit that will allow the use of a line level signal, but I'm not too confident about it.
Also, assuming that you had a "regular" aftermarket amplifier, you would still need to connect both the signal and ground to + and -. If you don't, you'll probably get a lot of hum. It may work, it may not, but that ground (-) is the reference for the music signal.
Did you get that 1/8" stereo -> Dual RCA jack yet?
I have one and I have a lot of line noise now.. I tried both with my laptop and a RIO mp3 player. Both had similar line noise.
If you splice wires into line level inputs, my guess is you'll get insane hum/line noise...
So my only option is to get an aftermarket amp??