Have you ohmed out the speaker connections.
Try to put the scope on the outputs to the speakers.
Open the amp to expose the Internal connections and
measure there (seem kind of far to go but it might tell
you something.)
From my install procedure for E91 VP kit. for Hi Fi system.
I use my meter to record initial values of speakers and verify that those values
appear in the right places . If anything goes wrong I have a chance of finding it.
Stock door speakers measured 3 ohms at the amp tightening screws
Stock under seats measure 2.1 ohms at the amp tightening screws.
You would use the amp connector.
Color coding for wiring is in .
https://www.e90post.com/forums/showthread.php?t=138949