I would suspect the head unit ground might be causing the problem (assuming
you haven't re-routed the RCAs beside the power cable).

BUT even running RCAs next to power wire 99% of the time causes zero problems.
Bad grounds, loose broken etc. are usually the cause of whine.

radio was installed, and the whine from the system is damned near
deafening.
snip
I just went to the vehicle to check things out, and here's what I found:
- the amp is grounded to the d/s wheel-well. The paint was not (as far
as
I can see) sanded away first, but a metal (piercing) screw was used.

BAD BAD BAD BAD BAD!!!!!!!!
1) First off the wheel well is typically just welded on, not a good place to
ground.
2) Paint needs to be scraped away to get a good secure connection.
3) Screws almost never (by almost never i pretty much mean never) make a good
solid tight connection.

This is most likely your problem right there. take it back and tell them this
is bad installation skills and raise absolute hell.

Les