I hate to say it, but it's VERY highly unlikely that the secondary ignition system could be a factor. Anyone that knows anything at all about automotive ignition systems would immediately point out that for almost a hundred years now, they almost all ground through the engine/chassis/body. So a short to ground is normal. Note that there's only one high tension lead (whether it's coil-on-plug or conventional ignition) and that the spark plug itself is grounded through the engine/chassis. In fact on older vehicles (I don't know whether or not it's true with later technology ignition systems) the ignition system actually has a positive ground even through the rest of the 12V system is negatively grounded! How's that for adding confusion to the topic??