An alternator generates AC current, doesn't matter which way it turns. The output is rectified to give you 12 VDC, ground will remain negative. So you would think it would be ok to reverse rotation. HOWEVER, the alternator has a fan, which is either located behind the pulley or internally. Most internal fans are omnidirectional, so rotation direction won't matter. Most external fans are either clockwise or counter clockwise.
Zavrti ga vani na stolu nekom i gledaj kako se ventilator ponaša, obično izvlači vrući zrak iz alternatora.