I think I will be shooting you a pm I am having trouble thinking of the best way to do this. My problem is if I sum them together I will have to cross the signal over again. I prefer to not use the crossover on the amp. I know some of the audio control units are alot better then the crossover units built into the amp. The other issue I was thinking about the less the signal has to pass through the better.
As far as tapping the wires I was thinking about just grabbing the signal where it meets the speakers. And running the signal from there to the processor. That way I don't have to tap into any speakers so for some reason if I have to return to stock it won't be a problem.
I am very familiar with car audio but this mustang there are so many ways to achieve the same or near the same results. It's driving me crazy. Lol.