Whatever you do, bolt it down - either to the boot floor itself or to a false floor made of mdf.
Don't want it coming through the seats and taking your head off!
What he said!!! I fitted mine and didnt have a chane to secure it, 2 days later had an accident and....
Thank god no-one was in the back! Now my amp and sub are stuck down with the extra-stong, extra-wide velcro you can get from halfrauds or diy shop, well worth it and can be removed if needing to get to spare wheel.
With fitting; open the doors and pull the plastic trim on edge of the carpet up using a screw driver or similar. There is space to feed cables through to the rear. I put power lead and amp remote wire on one side and phono cables on the other, have been this gives best results. I think I earthed it on one of the bolts that the rear seatbelts are bolted to the car with, that was easiest for me. Most people say to use the car bonnet release for the power cable, I found this a lil tricky so my mate who does this all the time drilled a whole into the bonnet-side through the passenger footwell. Conseal the wires and it should all look god and sound good too.
Tip: Before securing the sub down, test it in different positions to find the optimum position for listenting. Mine sits on driver side cos I mainly drive alone and that sounded best for me.