Determining amplifier input polarity

I have an 89 RX-7 convertible.  It has headrest speakers which have their own amplifier and indivdual volume controls.  I'm replacing the stock head unit and want to retain that functionality.  I have a wiring diagram and have deduced which four leads provide the left/right signals to the amplifier from the head unit.  Howver, from the wiring diagrams I have, it's not possible to deduce the polarity of the input signals.

Aside from hooking it up different ways and listening to see if the headrest speakers seem to be in phase with the other speakers, is that any sort of testing I can do with a multimeter to match up the polarity of the amplifier input leads with the output leads? (I do know the polarity of the output leads from the wiring diagram and from how they hook up to the speakers.)

No.  It's just a phase issue and you wouldn't see a difference with a multimeter reading.  There won't be any reading at all on the amp wires with no signal input, either.

With headsets operating alone it doesn't matter which wire is connected to the right or left.  But if you're a maniac about getting these things right (like me and a bunch of us ;) listen to an old stereo recording from the '60s when they put entire musical instruments on one channel.  With these it's really easy to pick right from left.  Play it through your regular system.  Then play it again on your headset and adjust the wires to match.