Check the plug going into the door. Make and break the joint. It might just be a bad connection.
Front speakers are 17cm 6.5 inch. For a proper fit get a piece of mdf and put the old speaker on it and draw round it. Cut it out with a jigsaw and drill the 3 screw holes that line up with the existing holes in the door. The using a jigsaw again cut a hole in your new mounting bracket for your new speakers to sit in.
+1 yeap, with a MDF primer as well.paint the mdf because water down the window will cause the mdf to expand and crumble.
I used ronseal garden furniture protector - it does exactl what it says on the tin!+1 yeap, with a MDF primer as well.