[Ally]

Have you earthed the amp as close as possible to the amp? with a nice fat cable onto a clean point.
Is the hiss with the engine off?
Assuming most amps are the same when i did mine i had to turn the gain right down, turn the volume on the stereo to around 3/4 then turn up the amp gain up until it the point where it would start to distort then back it off a bit.
Hope i'm not teaching you to suck eggs.