Here's a post from Titanic by Paul Richardson (I don't think you're on here, Paul?) that might help if anyone is still struggling.
Having received so much help from this forum I would like to pass back some newly acquired knowledge in the hope that someone might benefit;
With the engine running my original Panasonic radio cassette had recently been giving poor reception on FM. Following advice from a local ICE specialist I was told that weak hissy radio reception, as opposed to engine speed related whining, was usually due to a bad earth. I was told that any bad earth connection on the car could cause the problem, even those not associated with the radio. He did say that the aerial and head unit earths were the most likely culprits.
Incidentally the ICE man told me that if cleaning up all of the earths didn't help then I should buy another aerial amp from them and run it in addition of the standard fit item. The new amp was actually a VW part and plugs directly between the head unit aerial socket and the aerial plug. You then just add a switched power supply and you're away. Cost £8, looks like a 4 inch aluminium cigarette. I was surprised by his final advice. I was expecting to be told old the Panasonic was past it and I needed one of his super duper modern boy racer head units with a bigger screen and more colour resolution than my home TV. Maybe I caught him on an honest day and when it comes to head units they don't make them like they used to?
First I unbolted the aerial amplifier from under the dash. I adopted the traditional upside down head in the foot well posture. This reminded me of being at work for some strange reason! The aerial actually earths through the amp via a polished steel surface on the amp body that is held in contact with what appeared to be a gold plated section of body structure at the base of the weld stud. Gold plated? must have been the blood rushing to areas of my anatomy not usually brought into play this late at night. I added a new flying earth lead between the amp and car body and bolted it back together. This was a lot easier to say than do. I bolted the other end of the flying lead to another stud welded to the car body. I had no reason to suspect the original shiny contact surfaces were a problem, but since I had the opportunity to add a second earth I took advantage.
Next, I measured the voltage from the earth lead on the back of the head unit to the battery negative terminal. 40mV across the 2 points. Not bad I thought. So then I put a second cable from the head unit earth to my new weld stud and re-measured the voltage from head unit to battery. 4mV, better but surely not significant. So I switched on the radio and tested the reception with and without the second earth cable. If you unplug the aerial amp power supply this weakens the signal and shows the difference in earths more clearly. Low and behold the new earth made an audible difference.
I was on a roll now. So I fitted my secondary earth to the screw that grounds the head unit's aerial coax cable. Better reception again.
With the help of some cable crimps and good quality crimp pliers I made permanent earths to the head unit and its aerial coax cables.
Finally, the road test. Bingo, reception as good as ever.
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