Firstly, let me give credit to the www.talkaudio.co.uk forum where I found the information below, but I thought a recap could be useful here too.


My system:
Panasonic head unit with two sets of pre-outs.
Infinity Reference 5000CS components in stock locations.
Ratho (Unknown German make) 4 channel amp for front components.
Pro-Sound active cross over for the front components.
+ sub and amp for sub.
5AWG power cable (fused at 40 amps)
4AWG ground cable
Cheapest RCA cables from www.cpc.co.uk (4 pounds for 5m ones and 2 pounds for 1m ones)

Everything above is really budget stuff, in total it has cost me less than 300 pounds


Problem:
I have always had a lot of alternator whine with this set-up when the engine was running (i.e. a high pitched noise that changes pitch with the revs (it sounds a bit like a mosquito)). I always put it down to having crap equipment, but apparently, one of the most common reasons to get alternator whine is bad earth points for the amps / head unit.

I was pretty sure my earth points for the amps were up to scratch (I tried five different points in the boot and under the rear passenger seat (sanded down paint and all)) with no difference to the sound, but with a boot that looked like it had some strange disease.

However, there was one option that I hadn't thought of before (and never would have unless I had read the talk-audio site), namely the earth cables in the engine compartment.


Standard engine compartment earths:
The standard earths in the engine compartment are quite literally rubbish. They're thin gauge wire which is no good for stereo purposes. After replacing two wires in the engine compartment with heavy duty 4AWG cable, the alternator whine is all but gone.

The cables you have to replace are the following:

Negative battery terminal to car chassies:
The negative car battery terminal has a cable running to the offside wing. This needs to be replaced with a short run of 4AWG (I think my cable ended up being about 15cm long).

Negative battery terminal to engine:
This cable is a bit longer. The stock cable runs to the engine block and attaches close to the starter engine (under the intercooler). It's a right bitch to get to without removing the intercooler, but it can be done (but expect to get cuts and bruises on your arms). I replaced this one also, and used less than a metre of 4AWG cable.


Results:
After performing the above with the engine running I get no alternator whine if the dashboard illumination is off. With the dashboard illumination on, there is a slight hint of whine, but a lot, lot less than there was before.

My front components now also sound a lot louder (especially lower frequencies), which I put down to the amp now having a proper earth connection (it's as important (or even more important) than the power connection because all units in the system use the earth as a reference point, so all units need a good earth connection to avoid ground loops etc).

The dashboard illumination whine should go once I upgrade the head unit earth (if I can be bothered, it's not really such a pain any more).

When I was testing the system, I ran with the front components only and to be honest, the bass from the speakers was good enough to run the system without the sub (obviously the sub add's bass lower down, but it wasn't needed in the same way as before (where I had no bass from the components)).


Optional extras:
There is also a third cable that could be replaced - the positive battery terminal has a wire that runs to the alternator. This wire is as rubbish as the earths, so should be upgraded. Unfortunately, the alternator isn't that easily accessible, so I didn't bother with it. I think you need to upgrade this if you have a powerful system, but it should be fine if you're not into street bass.


Modification cost:
2m of brown 4AWG cable from Maplins (2.49ea for half a metre) - 9.96GBP
4 4AWG ring terminals from Maplins 1.19 for two - 2.38GBP
Heatshrink - 1.39GBP

All in all, less than 15 pounds. If you're on a tight budget, you can go for the red 4AWG cable instead, it is half price. I went with brown because I thought it might look nicer in the engine bay (as red is traditionally for positive). I'm sure you can also find cheaper terminals, but I just bought everything from Maplin's as they had everything I needed.


References:
www.talkaudio.co.uk where the mod is called "The big three" or TB3