Regular lights (incandescent) don't care about polarity or even AC/DC for that matter, they crave only adequate voltage - the filament will glow no matter which way it's connected... but the lil solder nubbin on the bottom is the positive (engine's white wire) and it's metal case the negative (black wire/frame ground).
LEDs are far more picky - wire them up wrong and they may
never work again...
Problem with the white wire is there's no real lighting coil on these engines, the output feeble - the wrong bulb may just act as a kill switch lol
This thread has a buncha good info: http://motorbicycling.com/f23/lights...e-no-7306.html