Okay, what year is the car and which body style? (coupe, hatchback, convertible)
The more info you can give the easier it will be for us to help you out!
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1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4 tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".)
Easy, you have to remove the rear seat top cover (over the seats) i think has 3 or 4 bolts, then you pull the rear deck.
The side panels make it a bit tricky but its easy.
I am trying to install speakers in the rear deck and having the same problem. I've searched the web for two days with no real help. I have a 94 mustang GT coupe.
I folded down the rear seats, removed the headrest (three bolts), removed two plastic screws, and tried pulling up the deck. It is not fastened by anything else, of that I am sure, but it is hooked real tight at the ends under the side panels. I tried gently bowing it and working it side to side, etc. I ended up creating a couple of cracks and a tear. Fortunately the damage is not very visible so far (a little glue will be called for though). I think the deck would have come out without damage if the car was new, but 10 years of weathering has made the deck slightly more brittle. I also broke off a metal clip in the middle while trying to maneuver it (not real important though). Rather than risk more damage, I would like to remove one of the upper side panels if I can (around the rear window). Any suggestions on this?
I'm curious why I can't find anything on the web for this particular install. Seems like such a popular car and would be a common upgrade.
On a side note, in the interest of maintaining pristine condition, where could I find a replacement for the cracked rear deck? I may be able to repair it, but if cheap enough, would just replace it.
You need to remove the rear quarter trim panels around the side windows and the rear seat back frame. After you remove the "head rest" there are two screws at the top and two at the bottom of the seat back frame. To access the lower two you need to remove the seat bottom as well, use a medium size flat tip screwdriver and insert it between the seat bottom and the floor support. Sklide the screwdriver until you meet resistance, pull it out slightly and move it a along a little further then push in. This will release the seat bottom latch. With all these items remove the rear deck panel will slide right out. Its a pain in the butt to remove all these items, but neccessary if you don't want to break anything more. Ford was stupid to put the speaker in from the top, but Ford does stupid thing like this all the time. P.S. I'm a Ford dealership tech.
Thanks-- I am going to go the route of removing all the additional trim. What a pain in the ass though. What holds the upper quarter panels in? I figure the seatbelts and coat hanger need to come off, but is there any other hidden fasteners?
The instructions at www.flemworld.com is what led to damaging the rear deck when trying to "bow" it to get it to pop out. Like I said, it may have worked when it was new and plyable, but after 10 years it may be too brittle.
removal of rear deck 
Can't get it out without breaking 
