So I haven't been able to draw anything up but I had one of those signal converters spliced into the rear speaker signal wires coming out of the back of the head unit and then ran RCAs from the converter to my amp to power my 2 12" subs. You can find wiring diagrams on here so that you know how to wire the converters. With this wiring you will still have use of the stock amp plus the use of any other amps you install and can all be utilized by the stock head unit.

One thing I found is that for some reason my wiring harness didn't have a negative for each speaker channel. So instead I ran the negative for each channel on the converter to the head unit ground. I wish I took pictures when I did this before I switched to my new head unit. It would be so much easier to explain.