Yes, the glow plug does not need a certain polarity to function but for charging it does. From all that I have seen the positive button end of the battery points toward the glow plug.

On the cap there should be some sort of spring to provide some pressure. When ever you see a coil spring, the negative side of the battery presses against it. This applies to most applications including: tv/vcr remotes, rx battery box, tx battery tray ect.

Ryan