yody

Been doing a lot of searches but haven't pinpointed exactly how to do this. I am going to install a four channel amp, new 6.5's in the front doors, and new 6x9's in the rear deck. I'd like to keep the factory head unit (yeah I know, I know, but I really like the factory unit and hate aftermarket).

I know I need a line converter to pick up the input, but do I take the wires coming from the headunit going into the amp, or do I splice in from the wires going out of the amp to the speakers. What color wires/schematic?

Also is it a major PITA to run new wires into the doors for the front speakers? And should I keep the factory tweeters in the door (mounted near the mirror)?

I'm mainly concerned with figuring out if I splice before or after the amp for my line input, so if anyone can help me with that first, that woudl be awesome. Thanks