Definitely depends on the car. The BRZ was an absolute breeze, so are the C5 Vettes. Never added an amp though.
The latest cars are all a pain in the ass for anything aftermarket, I'd bet. The majority of vehicle systems are tied in with the infotainment system. Not much I can do at all with the R. The system in there is "good enough". Even my old Kenwood unit is better than what's in the car, but it can't tie into all the vehicle electronics.