Run a fused (1 amp, 14 AWG) wire from a fuse in the fuse panel that only has power when the car is on to one side of a switch. From the other side of the switch run the same sized wire to the REM (remote) connection on the amp.
This allows for full control and you never have to worry about forgetting to turn off the amp. The switch is so you can turn off the amp at will so as not to give grandma a heart attack in the car.
You can find fuse taps at any autoparts store.