True, a polarising filter might help a little, cutting down on some glare. I used a polariser for the first time in anger (though I've had it for ages) last weekend.
It can cut out a lot of the glare on the cars - whether thats a good thing or not is another matter as it can make the cars look a little unreal almost.
I think the main problem is one of simple metering though - you are using some sort of automatic metering and the dark trees in the background are fooling your camera into thinking the scene is a dark one- and thus compensating by overexposing. I first found that at the Euros 2005 - the only way to get good photos with the dark wooded area in the background was to use the camera in full manual mode to stop the overexposure.. You can probably probably adjust the exposure compensation but I find it easier to just go fully manual.
I find shooting at tracks like Southport and even Lee Martins CML track to be quite difficult since the racing surface is black in areas, or at least was at southport. It would always overexpose and wash out the colours... combined with the fact that the trees had partial cover over the track made it a nightmare.