If you install a cage in the rear half of the car you'll never be able to safely carry passengers back there again. The seatbelts are designed to stretch in a controller manner in a collision and all the foam padding in the world won't help if someone's head made contact with a bar. Even if it is only in the rear half of the car it's likely to also be dangerous to the front seat passengers too if you try to use the stock three-point seatbelts which can't prevent your upper body from sliding out to the side.
If you're looking at a cage for use at the track be very careful to read the rules and regulations as bolt-in cages aren't legal for many sanctioned racing series. To provide strength needed to make the cage useful it generally needs to be welded to plates attached to the unibody. Also don't try to cheap out or buy kits from non-reputable builders.
Eric H. - '04 Neon SRT-4 (eBlue/S2) - '04 SRT-4 (blk/stock)
'92 Dodge Daytona IROC R/T --
'91 Dodge Spirit R/T
plus a couple other turbo Dodges, a Cobalt, a couple Focus, a wagon, and some other stuff...