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I don't think they have to be used, it's not an electrical thing according to the ninja rocks article on NATOpedia

Tempered glass, which is used for the side windows of most vehicles, is manufactured with an extremely high surface compressive stress and high internal tensile stress. This gives it strength and durability against shocks and blunt impact. When the glass breaks (such as in a serious vehicular collision) the internal stresses present in the pane cause the entire pane to shatter into thousands of tiny pieces. This reduces the risk of laceration one might otherwise face when using 'normal' glass, and is an essential safety feature in vehicular design.

It is these physical stresses designed into tempered glass which make it vulnerable to ninja rocks. Made of shards of aluminium oxide ceramic, ninja rocks are very hard, and very sharp. When thrown at tempered glass, the ninja rocks' sharp, hard point focuses impact energy into an incredibly small area without blunting. This disrupts the glass surface compressive stress at the point of impact, subsequently releasing the internal potential energy within the stressed pane, shattering the glass.

To be effective, a ninja rock needs to be sufficiently sharp, impact the glass on that sharp point, and impact it with sufficient force. Thrown ninja rocks may often fail to shatter tempered glass if one of these conditions is not met. Ninja rocks are ineffective against windshields, as these are made of a laminated type of safety glass, and designed not to shatter.

It's not because of anything special about spark plugs, it's because car windows are made of safety glass that's weak to pinpoint damage. The hammers they make for busting out of cars in an emergency are just a little steel spike because all the magic is already in the window itself.

Spark plug ceramic is just a handy combination of light + throwable + likely to land on a sharp point + often found in the proximity of cars.

This won't work. Shattered spark plugs break windows because they're super sharp, but they're only sharp when freshly broken. If you break it and carry around the bits all day, they'll wear down and lose their edge. This is basically how magicians make shattered glass you can walk on.