How to determine which speaker wire positive

Putting new speakers in 02 yukon. I looked everywhere an could not find which speaker wire is pos an neg in each door. For example front passenger has dark green and light green. Factory speaker doesn't show from its plug. Driver is grey and brown. Haven't taken rear doors out yet. Any ideas how to find out? Google it says it will tell you but always ends up not tellin?

Aren't the original speakers marked? If not:

http://www.doityourself.com/stry/how-to-test-for-speaker-wire-polarity#.Uo4bI-KJ4_8

dark green is neg, light green is positive

see attached

Not marked never understand why thanks guys!!