For powering it you'll have to run a power cable from the battery. As it's a leased car and a focus I'd recommend something quality in the 250-400watt range. For that you'll only need 8 gauge cable from the battery. There should be lots of places under the dash and on the drivers side to squeeze it through the firewall. As per the ground you can find a good spot on the frame if you take the side covers of in the trunk or hatch, there will most likely be ground wires attached to the frame around there already. As to hooking up the amp you'll want a line out converter or an amp with high level inputs that the rear speaker wires can be spliced to. Myself I prefer a line out converter but both ways work. With picking out what components to use I'd need to know what lmusic you listen too to recommend. I wouldn't use what jay_nar suggested myself. It is cheap but looks pretty low end too me, again it's about budget. Bang for buck I like the JL W3 with a good 300 watt amp. The subwoofer box, another reason I don't like pre made packages, is very important. A ported box will be louder but not the same quality as a sealed. The size of the box is important as a larger box will make the bass deeper but the sub won't be able to handle as much power. If you or one of your friends is good at woodwork I highly suggest custom building, it won't be much cheaper but will pay off big time in SQ. Just so you know there will be some cutting just mostly the speaker wire going to the rear speakers.