Hooking up amp with factory amp already Installed?

So have a 2001 durango and it has the infinity sound system installed. Head unit to 120 watt amp to speakers. I have found a head unit that will work with the infinity system and the steering wheel controls. Question is if I hook up new head unit and plug rca in for amp in trunk will the existing amp effect it? Should I rip out factory amp and find a head unit that will produce the same power as amp?