Sensible upgrade from non Bose system

[Oko]

I've been trying to find out some solid information on the non-Bose system, and this is what I've got:

Front speakers are amplified by the head unit (RNS-D in my case).
Rear speakers are amplified by the rear shelf speakers, which also act as subs. (The part# on them matches the BOSE rear shelf speakers, at least).

I've not yet opened up the doors to check on my assumptions.
But if my assumptions hold true, I think the easiest solution to get my plan of a tablet running the place of the RNS-D would go as such:
Plug the tablet via 3.5mm->RCA into a 2-channel amplifier, plugging the front speakers into this amplifier. This leaves two channels for the rear seats unamplified for the OEM speakers to do it. Because screw the people on the backseat. I'm driving.
This way, I would have little need to do any extra wiring beyond the small 2-channel amplifier for the front seats. Might as well make it a four channel amp, and plug in a sub too while I'm at it. Or just use an active sub and rob the signal from the rear speakers, installing the sub beneath the rear shelf.

Now, since I know nothing of this stuff, where am I going wrong?