The hiss is probably the noise floor of your cables/wiring being amplified by an excessive gain setting on your amp. You'll sometimes hear this on systems where an amps' gain is maxed out...instead of using the gain on the amp to make the signal loud, push a louder signal to the amp from the deck, and amplify it less.
For example, my system had a noticeable hiss in it when my mids+highs amp was turned to maximum gain. My deck volume goes from 0-80. My music was insanely loud at anything above ~50. I went to the amp and turned the gain down. The hiss went away, because my amp was no longer boosting everything coming down the signal wires (including noise) by quite as obscene an amount. Obviously, the music got alot quieter too - so, i went to the deck and turned the volume up higher. Now i get the same volume @ 67 as i used to get @ 50, but there's no hiss in the signal when there's not alot of sound being played.
You want the amp to amplify the signal your deck is sending -- make that signal stronger, and the amp will be producing more sound from the deck's signal and less from the overall noise floor of the system. If you really had the gain set high and a low signal amount going into the amp, you may notice a large improvement in overall sound quality from changing how your system is set up.
