I have the 04 galant with infinity system. I am interested in adding 1 maybe 2 subs to the system, but here is my question. I know there is a factory amp installed under the passenger seat, but is there room to add and subs to it, or do I have to get a new amp for this?

If it's anything like older Infinity amps, there aren't "speaker wires" per se running to it, but connectors that go to each speaker. You'd be better off with a converter and running a separate amp.