most nissan stuff is switched on the earth side. so when you are turning on the parker lights you are earthing one side. the other side generally has ful time + or just + when ignition is on. I guess parkers are the former (always +) so you can leave them on if you need to.
while this is safe, it means that if the earth wire to the headlight is damaged and touches bare metal somewhere, it will turn it on. I guess that is what happened.
anyway....if you want to work it out yourself grab a multimeter. you should expect to find continuity (0 resistance) between the negative at the globe and the chasis, even with the switch off (if you don't the light should not be on!). From there trace back the wiring until you find a break/damaged wiring...must be there somewhere. when you've found and repaired the wiring you should no longer have continuity at the globe unless the switch is on.
or, take the globes out and take it to an auto sparky at your leisure.