Why don't you just replace the factory amp with an aftermarket amp that has a remote level control, or an adjustable input gain (almost every amp has this).. Running an amplifier into another amplifier will cause voltage issues and likely extreme noise. Your 300 watt amp (75x4) is MORE than enough on its own to run the speakers in your car. Get the factory amp out of the system and you will be much happier.
---------- Post added at 10:46 AM ---------- Previous post was at 10:45 AM ----------
PM me the exact year and model of your car and an email address and I'll send you a wiring diagram for your factory stereo system, if you don't have one already from google.