Here’s the thing: you may be thinking that free-air subs are simpler to set up than standard subs, because they don’t need a box. This is not true. Free-air subwoofers don’t need an enclosed air space, but they do need a baffle. That means that the front of the subwoofer needs to be completely separated from the back of the subwoofer. If you just mount a couple subs in a piece of plywood and leave it in your trunk, you won’t get any bass. What you’d need to do is set up the plywood so it makes a sealed barrier between the trunk and the cabin, so that the front of the subs are playing into the cabin and the back of the subs are on the other side of the barrier in the trunk. Then you need to seal off any other gaps between the trunk and the cabin–ie, holes in the parcel shelf, openings through the sides of the trunk behind the edges of the seat, and so on. If the sound produced by the rear of the subwoofer cone can be heard in the cabin, it will interfere with the sound wave produced by the front of the subwoofer, and the bass will be reduced.
As for the power wiring, this needs to be connected to the battery terminal with a fuse located as close as possible to the battery. The wire runs through your firewall, through an existing grommet or by drilling a hole. Then you tuck it under the edge of the carpet along one side or another of the vehicle by pulling up the sill plates at the bottom of the door openings. Then it runs under the back seat, through your plywood barrier, and into the trunk.
The amplifier’s ground wire just needs to be connected to a solid metal part of the body or frame in the trunk. You’ll need to connect the amplifier to the factory speaker wires to get an audio signal; you can probably access the rear speaker wires under the rear deck in the trunk.