checked mine today, and i found kiss is boarderline. its just over 50%, the stereo flag is flashing back and forth and i had to manually tune it.
the best aerials are aways going to be loaded whips, ones powered with a wire runing up the whip. standard vw basic i think, they have the greatest gain.
i had one cable on the original aerial on the roof, which somehow become two by the time it gets to the radio, probably via a diversity unit. the diversity unit was probably incorporated into the design when they use screen aerials so the unit has a change to find a good signal and avoid multipath on fm. with a single aerial on the roof this becomes irrelevant as you only have one source to receive the signal, but if they didnt bother with spliting the aerial cable into two outputs all vw radio's would log fm2 as missing. so i guess it was cost cutting to provide the roof aerial over the screen aerial, but then oh balls, we still need the diversity unit otherwise all the radio's will throw a fault.