dieselpb

This has been racking my brain for a little bit. I plan on ripping everything out of my car and I can't seem to understand something. If I have an aftermarket amp with a sub and speakers connected to it, the amp interfaces with the deck through the RCA cables. Everything I read on how to replace the Nakamichi sound system, you have to take things apart and make your own wiring harness from 3 harnesses. What is the harness doing? shouldn't the harness only be connecting the deck to the car and the deck to the amp through the remote wire?

I'm sorry if this is a newbie sort of question, but I'm not sure if I'm making everything more complicated than it has to be.