Well,
I have an 1999.5 with a pioneer headunit.
I HAD the hiss sound, even when volume was at 0.
It was driving me crazy, but anyways. Turn's out I didn't need a filter or replacing anything.
All I needed was to have my installation re-done by another futureshop technician, cuz the first dude didn't seem to know what he was doing aftreall.

Now, It's silent and enjoyable.

 

the amp in my 98.5 is located in the trunk. mounted on the top left side