Please help with Amp Wiring to Factory System!
 

I currently have the amp inputs wired to the rear speakers in my 04 Lexus RX330. There is almost no bass coming out, I suspect the low range bass is blocked in these rear speakers.
Anyone know a good place to tap the factory system for a subwoofer amp input? Front speakers? Anyone know what color wires/where to tap them?

This is a Non-Mark Levinson system.
Any help is appreciated!