How To Add Boom To Your Factory System
I'm not sure if tampering with your factory setup is a good idea (compatability issues). You might want to check into powered subs. In particular, I purchased 2 Infinity Basslink (200w each) subs from crutchfield.com for my R350. Ran power to each sub from battery under front passenger seat. Used factory subs' (in spare tire compartment) left and right channels to run signal seperate to each sub. Mounted them on back of the 3rd row seats facing rear hatch for bass reflex (use enough wire to still be able to fold down rear seats for cargo flexibility). For convenience, they came with wired remote controls I mounted on my drivers' side side dash to customize sound after setting both Basslinks to half volume, zero bass boost, and lowest frequency cut-off.
The Basslinks combined with the 440w Harmon Kardon factory system gives me clean sound with enough power to rattle my rear when I want to! Help this helps you out...