I canīt believe nobody has suggested adding sound insulation.
When I moved my Alpine Type-S 6.5" co-axials to my current truck I also put some tarmat in the front doors. I put a double layer on the outerskin to prevent vibrations and block out road noise. I put a single layer on the inner skin to prevent vibrations and to make the door tighter.
By closing all the holes in the inner skin your woofer will provide much sharper and detailed midbass. If the backside of the speaker is not well sealed from the front side you will get less than desired performance from your speaker. If the soundwaves from the back react the front you will get cancelling, the soundwaves will cancel each other out to some degree.
I noticed that my midbass was much sharper then in my old truck and there was not as much rattle when I turned it up. I also greased the rods for the locking mechanism so they stopped rattling. I also have extra insulation on the bottom/floor of the cab, this is a mat that is laid under hardwood floors, its a rubbery poly something. It works great and is easy to take out and put in.
I plan on putting tarmat in the rear doors as well, back of the cab, top and floor. This should make it much nicer to daily drive then the all vinyl interior I have now.
