help with sub/amp install

i have an 02 x and im going to run a sub and amp today for my first time and my question is.. am i going to need an aux hook up to the factory head unit? i already have an amp running to it for my speakers but had a company intall it. thanks

The simplest way imo is to get a Y-splitter for the RCAs.

does that plug directly into my head unit?

whoever installed your first amp should have everything wired up with RCAs. At either end of the original RCA, you split it for your second amp.