99 Lexus GS300, factory amp

Looking for a little help on intergrating an aftermarket radio with the factory amplifier, speakers and sub. Have read some of the past posts but nothing has mentioned keeping the factory sub and amp working. It is very easy to wire out the speakers and the the head unit to power those but the factory sun and keeping that is not so easy. My next step is to try and wire low level or high level into the factory amp,nothing working at the time and also no specific amp turn on wire is to be found. So with that I can keep everything wired factory other than just the headunit. The factory radio is manufactured by Pioneer and not Nak. Any help would be great or if you have come into the same situation.

It looks like the whole problem could stem from the Bus+ and Bus- wire going into the factory amp. Without those two wires connected to the factory radio nothing will work. As soon as those two wires are disconnected from the amp, the amp shuts down.  Any Ideas?