I have a laptop in my trunk which I'm using as an MP3 player. Up until
recently I used a cassette adaptor to pipe the sound into my 2000
Passat's factory head unit. I decided to upgrade the sound quality a
bit, and bought a PIE VW-AUX (a device which converts the unused CD
Changer connector in my trunk into an auxiliary input) adaptor and now
have the audio from the laptop running through it to the head unit.

Everything works like a charm as long as the laptop is running off its
own internal battery. If I select CD1 on the head unit, I can hear
nice clean(ish) audio from the laptop.

When I have the laptop connected to the power inverter (plugged into
the cigarette lighter socket in the trunk), though, I get this really
loud buzzing/humming sound coming through the speakers (lound enough to
almost drown out the music completely). It doesn't matter whether the
laptop is on or not - as long as its plugged into the car battery
through the inverter, I get a loud humming sound when the CD changer is
selected on the head unit. About 5% of the time, the humming sound
might go away for a little while, but always comes back again.

I did some research, and it sounds like I have what's called a "ground
loop" problem. However, I've tried two different ground loop isolators
(one was made by a company called Stingray, and the other was the PIE
EIS-ILNO, which is supposed to be both a ground loop isolator and noise
filter) and neither one has had any effect.

I've tried a different power inverter, but the humming sound is still
there. If I'm in the garage and plug the laptop to house AC current
the humming is gone (although this isn't a viable solution for the
obvious reason ;-)

Is this a ground loop problem, and if so why aren't the ground loop
isolators making any difference?

If not, what is it and how do I fix it?

Is it "safe" to leave things in this situation? At the moment, I have
the inverter on to charge the laptop battery only when I'm NOT
listening to music through it and switch the inverter off when I'm
listening to music. However, the radio is on when the laptop is
charging so whatever is causing the humming is still coming in through
the CD input even though I'm listening to the radio. Is this harmful?
Thanks!