Daniel K has a good answer, but still remains the doubt that the wires may be reversed at the radio side.
Another test is to play a sound rich in lows, listen carefully, switch the wires, compare sounds.
When one speaker is reversed respect to the other, there will be some low frequency cancellation, but it may be hard to tell.
If it sounds good to you, one way or the other, just enjoy the sound and don't worry about the polarity.