I'm assuming you don't have an aftermarket amplifier? What happens when you set your radio fader to full rear? When I got my 99, everything would go silent. That means that the rear output on the radio is not currently powering anything, although I'm pretty sure the wires are still run from the radio to the amplifier.
Take a look at the manual for the radio:
here. Page 28 has the pin-outs, and it looks like socket C1, pins 1 and 2 are your left and right rear pre-outs. You could tie into those two and the ground and feed them to the un-amplified input on the sub.
Getting power back there to the sub is another challenge...
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