Wm caprice amp alterations

[TrickyRicky08]

I have a wm caprice. The stereo system isn’t quite good enough for my liking but I’m trying to avoid going crazy on enhancing the sound. I’ve replaced the speakers in the doors which helped considerably. I’m going to put a pair of subs in the boot with a alpine mono amp as the bass it has now is terrible(my factory amp doesn’t have the gain knob on it that a lot of people talk about).
I’m curious if I can take the existing power from my subwoofers to run the rear speakers?? I assume that then I would use the rear speaker output as a input line into the alpine amplifier.(it has the speaker input lines on my alpine amp.
Any advice please, cheers.