input

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.

Warning this question will probably sound extremely dumb. So in short, if I have an OEM headunit and i use a line level converter to output to rca and then used a rca to aux 3.5mm adapter. Could I use that output for an aux input on another headunit? Thanks.