For those who don't want to solder (I stink at soldering), this is an option.
I put these in my Jeeps. You basically plug this in between the factory antenna wire and the radio, run hot and ground wires, and when you want to use AUX you turn your radio to 88.3 and the AUX is piped in that way.
https://www.amazon.com/Scosche-FM-MO.../dp/B001QBG614
Works great.