polarity

Just a heads up people. I just finished installing a new component set. I made sure it was all wired properly, positive to positive and negative to negative. I wanted to double check everything with free polarity tester app from the iOS store. The new components all tested fine but my existing...

Can you use a multimeter to determine polarity on wire that is unlabeled (potentially while music on)? Wires were connected to factory rears to run to amp high level in. Those bare wires will power a test 5x7, just unsure of polarity when wiring my high level adapter.

I am confused from different sources. I have 2 inverted 15s wired normally(positive to positive, negative to negative). They sound good. (Video of them: https://youtu.be/VoQzsELrfCc) However i have heard when you invert the subs you have to invert the polarity as well to the amp. (Positive to...