First of all, do you have a drill, and are comfortable removing interior panels, seats, and the like to run the wire to the trunk>? You need to run a power wire from the battery ( locate the fuse within 18" of the battery, preferably as close as practical ) to the trunk.
Then you need an ignition controlled wire run to turn on the amplifier, when the ignition is turned on. You also need a line output converter to feed the amplifier an audio signal, this can be tied into the rear speaker wires in the trunk. You can ground the amplifier to a spot on the chassis close to the amplifier in the trunk.
If you aren't comfortable doing all of the installation, it will cost you more than $ 200 in labor to have the job done properly, not including the parts. At a minimum for most small amplifiers, you are going to need ~ 15-17 feet of 8 gauge wire ( red for positive ) as well as 2-3 feet of black wire for ground, 20 feet wire loom ( 1/4" - 3/8" ) electrical tape, grommet for the firewall ( assuming there are no blank open ones ) a fuse block and fuse, tie-wraps, etc.
You can buy used gear, but if you don't know what you are getting into, you could end up spending a lot more than you planned to get what you want.