I certainly get that. And if I were using an aftermarket headunit, I would certainly do it as I've done traditionally. One of the test I did when playing around with the bare harness was to play the rear speaker output to the front just to check out the output range (by ear as I don't have an RTA). It seems the output is pretty EQ-limited and would benefit greatly from summing the channels and optimizing for the installed speaker. You may be planning this with the amps you're installing, but I've started down this Key path already.
This should help anyone wanting to build their own affordable RTA
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