That cable may or may not be correct. Typically audio cables have red/white RCA connectors on the end to indicate right/left channels. Yours has a black and yellow one. Yellow is typically composite video (aka TV-out).
As for connecting the 6 speakers, I don't know. I'm not the car audio guru. But I'd think that a pair of matched components with their own crossover would work as one set of speakers, but that may or may not be what you want in your ride.