These particular Alpine amps are their 2nd gen digital, they require less power and put off less heat in that small package. They are just the way I went, it's not the absolute right way.
Just make sure you have lots of power for the sub(s) of your choice. My subs are 250w a piece so I'm driving them with 600W.
You need a summing unit, that takes all the signal from the various stock speaker lines. And I think this has to be done from the stock amp it actually has the brains built in it for the head unit.
ZXSUM8 | KICKER
I have the kicker summing unit, but there are various brands, your installer of choice will have a favorite that he knows how to use well. Just choose one that doesn't require it's own volume control, that would defeat the nice volume controls on the steering wheel.