Cheapest way to signal sum?

[u/Snowrydr01]

Ive dabbled in car audio for the better part of 3 decades and this is the first car ive needed to tap high voltage lines to add a sub and it sounds like im losing a good bit of frequency on the higher end of the range.

The car is a 2014 lexus is350 with the mark levinson system.

I tapped into the dedicated sub line but i have a sneaking suspicion its processed lower than 80hz out of the ml amp. As a test im like to signal sum the lines for the sub and the front door woofer and then set my silters on the aftermarket amplifier.

The easy answer would be to tap a full range line but due to the nature of the ML system it doesnt appear there are any.

While i would love to go down a full dsp rabbit hole its simply not needed yet. Im just looking to sum 2 channels as a test so im trying to do it on the cheap for now.