Alternator noise over speakers

[rplines at pacifier.com]

I just dealt with this problem on my 4kq.  When I installed a new head unit, I got
the power supply from a "spare" fuse in the fusebox.  The PO had run a wire direct
from the battery (it was a hack job, no fuse, etc...).
After I moved the power supply to the fuse box, I started hearing alternator whine
through the speakers.  I cured the problem with a Radio Shack inline noise filter
that attaches to both the power and the ground wires.  I'll try to show it with some
ASCII art:
                     ______________
power supply---------|             |-----power to head unit
                     |   filter    |
                     |             | 
ground to chassis----|_____________|-----ground, head unit

I'm not sure why moving the power supply to the fuse box caused a problem, but obviously
the battery was a cleaner power source.

-Peter