The bottom seat you have to push on each side halfway between the driveshaft tunnel and side of the car. You have to push back toward the rear of the car and up and they'll pop right up. The rear uppers as mentioned you have to undo the phillips head screws on the rear covering the hinges and then take (i believe a 13mm) to take off the bolts. All that stuff should come out pretty easily. My car came without a rear seat and i also took out all the sound deadening material (including the tarish stuff underneath). The car is probably 100 lbs lighter than stock. I'd recommend it, especially if you want an excuse so you don't have to carry all your friends around in your car (i guess i've saved alot of gas because of this as well, not having to drive as much).
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Mustang Parts Specialties ripped me off, it could happen to you as well.
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT