[Gogar]

some amps have a 'speaker in', where (if the sub/amp is in the trunk) you can run the wires from the rear speakers into the amp.

You just buy 4 ohm or 8 ohm speakers depending on how you plan on wiring everything and which amp you pick. If you're going to have just one speaker and 'bridge' the amp, then buy the 4 ohm speaker; the bridged amp will see a 2-ohm load, which will give you the most bang for your buck without blowing it up.

If you want to use 2 speakers, buy the 8 ohms and wire them in parallel, = 4 ohms and run them into a bridged amp, = 2 ohms, = proper (depending on the amp, but most likely fine.)