I have a stock system and I am currently in the market for just the infinity mopar upgrade to keep it simple. My question is, If I get a system from someone is there any additional wires that are extra for this?

 

I would caution you to even bother upgrading to stock parts.... they will sound just as bad if not worse.
You having the base system are better off. Get an amp with speaker level inputs and then output some more powerful sound to those speakers. Replace the speakers and have it sound better. Add a sub and enjoy more complete sound.

Upgrading to the stock "premium" system is a HUGE waste of money.