89 hatch door speaker quesiton

I dont have any plastic behind my speakers and I havent had any problems. Im guessing that the door has some sort of internal run channel. The speakers are really close to the front of the door also, Im guessing not a lot of water makes it up there. You could always put some plastic flashing up there to make totally sure if you want. I proabably should now that Im thinking of it. It would be good insurance in case some water does get in...
 
Oh, as for the wires. I would reccommend running new ones for the sake of simplicity and conformity. If you use an amp for you speakers, you WILL have to run new wires. If you are plugging your new deck into the factory harness and just running the speakers off of deck power, then no. In that case, cut the connector off of the factory speaker plug an solder or crimp one on that fits your new speakers. I like running my own wires because they are easier to work with. Plus I FEEL like it should sound better because I spent so much time on the install... More satisfying!!!