Capacitors or bass blockers are used to as a first order low freqency reducer when used in series with a resistance such as a speaker. It's a cheap, but effective way to keep a speaker rom outputing low freqencies that it cannot effetivly handle.
About 8" woofers in the rear deck, I have one where the 6.5" hole is already. A second one would be much more work. Anyway, the speaker was only $50 so you shouldn't have to save up too long.
I'm sure Dave will pop in with more specific details for caps and free air thughts.
