PC provides very low level inputs (On the scale of 0-0.5 or 1volt). Generally lower than most amps are rated for. I can't speak for Factory amps, but most amps you buy in the store have low level inputs as high as 8v in some cases. You need to find out, perhaps from a G35 forum or look up the actual part number of the amp, what kind of inputs the amp takes.
I would assume it is expecting low level inputs, but factory installed amps can be all sorts of weird things.