Adding amplifier & adding equalizer - wire diagram for amp

Adding amplifier & adding equalizer - wire diagram for amp

1) Can someone post the wire diagram for the factory amp or direct me to where I can view one?
2) Does splicing into the low-level / line-level wires degrade volume of existing speakers?

I want to add a single amp to drive a sub. I also want to add a passive equalizer on the speaker-level output wires to filter out some of the mids. I'm not looking for a competition worthy setup. I'm not looking for a $million solution. I've done a search. Someone asked the same question about adding an amp and sub, but the thread ended up being a showdown of sub brands and question was never answered.