check your speaker with 9 volt battery and wire, I took an old toy and robbed the 9 volt connection off it and hooked red and black wires to it. then you can just touch you speaker terminals to see which is pos+ and which is neg-
Usually there is one solid, one striped. Striped is positive most times. But not always. Do you have a DMM? Probe the wires, it will tell you. Meter has red and black wires. If you put the red meter wire to the solid speaker wire, and black to striped, meter should read 4 volts or whatever. If you hook the red up to the striped and the black to solid, and meter reads - 4 volts, then the striped is positive.
exactly as mark said a digital multi meter. your best tool, on my old truck it had that type wiring and the blue or darker color was m positive, but don't quote me as that being your case. use the meter all will be peachy.