I'm not familier with this speaker it is different than my XK. But me not wanting to shy away from a challange. If you cannot see and screw's on the grille cover then it is a friction fit. In my XK I can just pull up and the grille will come off. You should be able to pry this off using a couple of putty knives. I would get a 6" and a 1". Place the 6" flat on the rear deck as close to the grille as you can. Then use the smaller to get under the grille. What you are doing is prying against the wide blade so you wont damage the deck board because the wide blade is protecting it. If you need more leverage between the two knives you can us a finishing nail between them. When the grille pops off you should see the screws. The next problem will be can you get to the screws with a small enought screw driver and not hit the rear glass. If you can't Milwaukee Tool makes a 90 degree attachment for your drill, official name is
(Offsett Drive Adapter 48-32-2100) that in most all cases will allow you to get the screws out. This is a great tool to have regardless.

I bet your sorry you asked now aren't you!

 

Have you tried contacting Harmon Kardon direct. Also you my try an independant custom stereo shop in your area. I would look for one who does dealer work for custom upgrades. Your local domestic dealers are usuall the ones who may upgrade new cars.