Ok, test is over and you've probably figured this out by now:
White wires are positive.
Brown wires are negative.
Quick way to tell:
The connector in the stock 6x8 only goes in one way, and the polarity is marked on the stock speaker. (This is true at least on the speakers that came in my Merc, which I have LONG replaced.)
Rob-o
HOME OF THE MERCRUISER
and it ain't no boat, baby!