Hiss, i hate that, especially when passengers notice it and go, 'whats that sound'
Anywhos... i had this problem and found that my wires running from the head unit to the amp were running with power cables. When running cables try to isolate them from other cables, esp. powered ones, as much as possible. this will minimise hiss.
Hopefully you can upgrade and employ some RCAs, and dont go cheap on the RCA's either. get some shielded ones.
If you were like me and still had a slight hiss when all was hooked up, try using a filter on the inputs at the amp end, or Run a wire from the grounding point of your amp back to the grounding point of your headunit. This worked for me.