Yeah thats what I found When I started looking into adding amp and subs so this is what I used: http://www.reqsound.com/A word of caution about the line level converters. The monsoon amp attenuates the bass to keep from blowing the speakers. When you have a converter on the 6x9's, it works, but as you turn the volume up, the amp turns the bass DOWN. If you have a remote bass knob, you can probably compensate, but I didn't have a knob, and could never adjust the amp gains to play hard at high volume OR they'd bass me out when it was turned down. Best way around this is to tap the Monsoon amp inputs, as this is pre-EQ. I picked up a Radio Shack RCA speaker cord and cut it in half.
Hope this helps!
Made it easy just spliced the rear speaker wires and got it for 80 bucks off amazon.