Following my install this weekend I can only suggest the following:

- connect a braided battery strap across the hinges of your bonnet to ensure the bonnet is properly earthed. This will form a good Faraday shield and reduce radiated field strength.

- check the alternator connectors are clean and greased with white vaseline.

- fit the VW suppressor kit that is available from the dealer. This includes the HV leads; not cheap, but worth it. My Polo was much improved when I did this years back.

- ensure that the filter supplied with the empeg is used, inline with the power feed to the unit, and that it supplies ONLY the empeg. If you saturate the coil with a high current, it will be affected by interference.

- measure the earth path resistance from the amp to the battery negative terminal; if it's more than a few milliohms, then the return current from the amp when operating at high volume will cause a voltage rise across the cable and jack up the earth reference for the amp.

- if possible, reduce the pre-amp input gain on the amp.

You describe the alternaotr whine level being randomly variable; this could be associated with battery charge state, ie. a flat battery will cause the regulator in the alternator to jack up current output. Most German cars use open-cage air-cooled high voltage rotors for compactness; trouble with this is, the regulator has to work harder. They have changed a lot of traditional designs from a rectifier bridge and regulator to a direct chopper/switching regulator setup years ago - this was started by Bosch, and quickly followed by AC Delco. Lucas stuck to analogue for a long time, although I'm not sure what they do now. The open cage means radiated field, and the switcher means interference.

So what can you do about this? The first and simplest thing I would start out by doing is looking at your battery state. Measure the voltage without the engine running, and for a 6-cell lead-acid you should be seeing between 11.5 - 12.5 V. If it's any lower, then your battery may not be charged or is low on cell electrolyte. It will not be able to hold charge well in this state, so you will need to see if you can get it topped up and re-charged. Alternatively, you could invest in a new battery of a higher Amp/hour rating than the one you have now. I recommend Bosch, but you can look for any make that has *large*, thick electrode plates, not tiny thin ones. When you install the battery, make sure that the terminals are greased with white vaseline, and that the earth lead is in good condition, with no corrosion around the contact point to the bodywork.

Now listen to the installation again and see if there's any difference. If not, then I would suggest you trace you power feed cables to where they are connected to the charging circuit. Does it go through a fuse? Check the fuse is clean and has not corroded it's contacts by rotating it/removing and re-inserting it. Grease up the contacts again. Measure the battery voltage at the battery, and then measure at the power feed to the empeg in-line filter. If there's any major discrepancy (around 1V or so), then you have a high-impedance power feed which will have to be re-done.

I can't think of anything else off hand other than manually moving connectors and cables while in motion one at a time to see if there's any effect. You could also try rotating the phono connectors through 90 degrees to see if orientation has anything to do with it.

Good luck - I would bet on a combination of corrosion, battery, and bonnet.