Hey buddy, that’ll be a tidy little setup you’ll have there. Usually when I’m doing crossovers in a Commodore, my crossover gets mounted in the kick panels of each side. This is also the easiest place to intercept the wiring. There is absolutely no benefit to running new wiring from the Headunit to the door speakers. I tend to intercept the speaker wires at the plug leading into the door. I’ll extend them a few inches so they can easily feed into the xover and then just run a new wire straight up to the new tweeter instead of trying to resuse that wiring. On the headunit, you’ll have the ability to apply a HPF (high pass filter) to the door speakers and a LPF (low pass filter) Do so. This will remove the bass from the door speakers and the high frequencies from the sub. Allowing the speakers to only deal with frequencies they are designed for. I’d probably try setting the HPF and LPF to somewhere between 80-100Hz. Choose whatever sounds better to you.