Originally Posted by Jayhovah

The speaker output from the headunit will go to the input of the cross over.

If you dont want to hack up your harness, you'll only be able to use the OEM wire for the door speaker or the dash speaker, but not both. The OEM speakers have their individual crossovers built directly on them, so the wires for each side actually run to both the dash speaker and door speaker. (i.e., the drivers side speaker positive wire runs to both the positive side the drivers dash speaker AND the positive side of the drivers door speaker)

The easiest configuration is to connect the woofer output on your crossover to the OEM wire and use it to for the door speakers.. then run a new wire for the tweeters.

Hope that helps!