The OEM dash unit doesn't have low level phono outputs like the aftermarket units all do but all remote aftermarket amps have optional speaker level inputs so you can tap into your existing rear speaker wires to get the sound to the sub amp.
Now, as for powering the sub amp, they use alot of current so you'll need to wiring a power source to the battery with a heavy guage wire and good high current fuse located at the battery. The amp is switched on/off usually by the antennas power lead (low current) and you've already got that back there now.
Places like BestBuy, Circuit city, etc sell amp wiring kits if you want to simplify the install but some of those kits get kinda pricey IMO.
If you'ld rather not install it yourself, those places will also install their stuff in your car too.
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