Aftermarket amp install

I have a 2000 sebring with the infinity sound system and want to add an amp and some subs. At this point I don't want to replace the HU so I plan to use a hi/lo converter, but I'm worried that if I tap directly into the rear speakers that the wattage may exceed the limit on the converter. I thought it might be possible to connect the converter to the wires coming into the amp to avoid the increased power, but I don't know which wires are for the rear speakers. Secondly, I would like to tap into the remote turn on running to the factory amp ( located in my trunk) but I'm not sure which wire is the remote turn on. There are several with white and blue coloring or even black and blue. Thanks for the help.

When you say Hi/Lo do you mean Low frequency pass filter? I would create a center channel output by tapping off the left and right rear then run that to the sub. I'm not sure what wire is the amp turn on.