As far as I am aware, the stock system does not provide for the cross-over, but I am not sure about the HK system. Regardless, you should replace the Crossover if there is one as they would not be of the best quality.
As for the door panel, behind the door pull handle on the inside of the door are 2 little black abs that need to be removed what cover 2 bolts. You also need to remove the plastic clip that surrounds the surrounds the door handle. It slides off. They are pretty easy to break, so be careful. You also need to unscrew the plunger from the top of the door.
To get it off, there are clips that go all the way around the door. You need to pull the the panel off the clips gently. The panels are made of some cheap MDF stuff or something and can crack easily where the connectors go in.
Hope this makes sense.