this would cost you almost as much as buying an inexpensive aftermarket amp to power the sub. the factory premium amp is not ready to be bridged as it stands, but you could put together a little circuit that acts as a bridge to keep the signal stable. then you will need some kind of low pass crossover to keep the high frequencies from being played through the subwoofer. you should also build a small impedance matching circuit into this crossover to keep the amp from trying to drive too much power to the sub. it might clip quite readily. and after you have done all of this, you will have a modest (at best) power supply for marginal bass...since i'm sure the factory amp is not rated through all of the bass frequencies. it probably nominally operates until 40 or 50 Hz.
i say, go aftermarket and scrap the factory rig.
