The factory stereo's crossover is the little capacitor on the tweeter.
You can house the cross over under the dash and run wires to your TW and through the door gromit to the woofer, or place the crossover inside the door panel, run the tweeter wire through the door gromit and into your tweeter.
Eitherway works but you need to run wires from the cross over through the door gromit at some point because there's not only the crossover for the tweeter, there are choke coils for the woofer's F0.
GL!
