[djrobx]

You don't really need to know the polarity. You can work that out by experimentation; you're probably going to need to anyway.

1) Connect only one of the the two underseat sub lines to your amp/LOC/aftermarket sub and play some music with the sub on but playing at a medium level. Make sure to close the trunk hatch and position the sub roughly how you intend to have it. Sit in the driver's seat and listen. Is the bass stronger or weaker? If stronger, your polarity is correct. You'll know if it's wrong, the bass will sound really strange and probably weaker than with the sub on. Try both ways, one will clearly sound better and stronger than the other.

2) Once you get #1 right, connect the second underseat sub connection to your amp (I'm assuming your amp will turn the stereo signal into mono properly). If the sub practically stops playing, you know the polarity on #2 is backwards. Just reverse + and - and you're good to go.

Nothing will be damaged by getting the polarity/phase wrong. It's just a matter of your add-on sub canceling out, or augmenting the the sound from the factory system. FWIW even with my harness, I had to hook my sub up with the + and - reversed to get the phase to match the under-seat subs correctly.