I am installing an amp in my 1986 Porsche 928S. Because the body is
galvanized and treated, there are only a limited number of choices for a
ground.

The setup is fairly simple. A head unit (Kenwood MP922) to the amp (American
Legacy LA660, has a 30amp fuse, "1000W") by RCA cables. I am running a short
(~3 foot) 8-gauge wire to the primary ground strap in the rear of the car.
The battery is also back there, so amp power comes from another short 8ga.
wire to the battery. Head unit power comes from the stock harness.
Originally I had the HU grounded to the stock harness, but in order to
eliminate alternator whine, I ran the head unit ground all the way back to
the same place (on the other side of the car from this ground wire). There
was no change. It's *extremely,* loud at idle. Speaker wires tie into the
factory wiring at the old factory amp location on the pass. side floor.

So I checked the google archives of this group to start to isolate the
source. There's no whining sound when the RCAs are unplugged from the amp. I
think it gets progressively better as I unplug them but I don't think the
effect is large. A walkman running on batteries plugged into the amp gives
no whine (sounded shockingly good, lol). I then installed a power filter on
the HU. No change.

I don't really know what else to do. I get whine from a "local" ground and
when the HU and amp are grounded to the same place. What's left?

Thanks for any advice...