Amen, brother. Again, what's the point of a rear seat delete, if not to lose dead weight? If your answer is "looks," think about that a moment longer. IIRC, the first Cobra R's to come out with things like radio, A/C, and rear seat deletes were not for looks or luxury, but to shed pounds (and save costs, I guess). If you go about deleting the rear seat for the sake of giving it that more "hardcore" look - like, "Hey, my car's got a rear seat delete, too, like a Cobra R!" - but you're also adding 30 pounds of wood and carpeting n' such in the process, you're basically just shooting yourself in the foot (performance-wise) and being a poser (to anyone with a clue about what makes cars go faster). Not that the Cobra R's the only example, that's just what readily comes to mind.
If you install a rear seat delete just for the sake of making room for another 200 pounds of audio equipment (as well as the extra 20 to 30 pounds from that $300 delete kit), maybe it's time to reprioritize your wants and needs. If you want a ghetto cruiser to go bumpin' around your beats and lookin' fly, yo, then do like every other G-money thug out there and buy an old boxy GM car (Caprice, Regal, Monte Carlo, whatever), throw some 20" bling-blings on there, install some hydraulics/airbags, and fill that huge trunk with all the batteries, solenoids, air tanks, and subwoofers you can get that thing to haul.
A Mustang is a PERFORMANCE VEHICLE - a car built with an emphasis on acceleration and handling. Just the same, cars like Civics and Tercels are ECONOMY VEHICLES - cars built to go from Point A to B with minimal fuel costs and a relatively low initial purchase price (or at least they USED to be cheap). Trucks are UTILITY VEHICLES, and big ol' boats like Lincolns and Caddies are LUXURY VEHICLES. They all have their specific intentions for use in design, build, and marketing. Sure, you can take any one of those vehicles and try to cross-train it, so to speak, to perform other roles by modding the hell out of it ... but WHY? None of those vehicles of other classes will ever out-perform another vehicle of *equal modification* in its original class; you're starting from a point of disadvantage and setting your sights low to just equal another car's stock abilities/benefits, rather than expanding upon a vehicle's original intended role.
You wouldn't take a Geo Metro out to go in a rock-crawling competition against a bunch of Jeeps and Hummers, would you? Nor would you realistically want to take a 5000-pound '72 Cadillac Fleetwood Hearse to a drift competition, right? So, why on earth would you want to take a perfectly good Mustang and load it up full of crap just to make it look/sound pretty? (Please, don't use the "I want to be different" or "expressing individuality" lines. Even Emo kids are resorting to those to explain their goofy antics, nowadays, and it's just laughable.

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Anyway. To reiterate, $300 for a rear seat delete = DEE-DEE-DEE. Build one yourself, or have a metal shop fab up a nice sheetmetal/aluminum one and then paint it to match. It'll look dandy, cost much less, and work far better for its intended purpose. At the very least, shop around a little for a better-made one on eBay or something - I've seen a couple of places online (not necessarily all on eBay) with lightweight delete kits that're NHRA legal and still reasonably priced.
Oh, and to return to the original poster's questions: The rear seat is a two-piece gig. The lower half should just pull out by hand; you might have to push toward the rear on the car a bit to get it to unhook from its base spots, but there's no tools required there. You'll see the bolts that hold the top half in place, once you get the bottom part out. I dunno if they use the same Torx bolts on the top half of the seat for hatchbacks that they use for the seatbelt mounts, or if they're just plain bolts, but regardless, you'll need a big ol' honkin' Torx bit (T-50 size) to remove the seat belt mount bolts. Once those are out, start scraping that tar-like poo out from under there, and spray some rattle-can paint or whatever you desire over the remaining primer to seal things up and make it look nice. Then, proceed to the planning/design/fabrication/installation phases of the rear seat delete portion, itself.
Enjoy!