true or false amp mount?

So ive always screw the amps on the trunk, specially when there fold down seats there really easy. But this time ive got a chevy impala ss 96. So i got all the stuff working properly subs, head unit, and speakers. Next day he takes it to a diffrent shop to get his window tint done, and I guess they told him that the amp cannot be mounted to metal witch is the back seat. First time hearing that whanted to make sure its true

As stated above, there were some older amps that would give you noise problems if mounted to the chassis of the car.  However they all put rubber grommets on the mounting holes so the screw or the chassis of the vehicle were not allowed to touch the chassis of the amp.  Linear Power is the only brand that I can think of right now.  There were several others.  These were all from the 80s.  Lately I have not seen any amp that was not to be mounted to the chassis of the vehicle.