Sounds like you got the idea, the amp in the car is 4 channels, the front door speakers and door tweeters are on the front 2 channels and there 2 bass speakers behind the seats are on the other 2... All I did was add a small tweeter amp for the door tweeters so I could use the low level out puts on the stereo this gave me more control over lows, mid’s and hi’s... if you are just adding a bass in the back it might be easiest to just ad a amp to the head unit off the bass output... But you could set it up anyway you like from adding to the amp to using 3 new amp, its all up to you... good luck on you project!
Mira