There's a little circuit board you can get, microprocessor Controlled 12-volt automotive personal computer shutdown/startup controller, (mouth full), which basically after you swith off will hold power to unit in order for it to shut down fully, so you don't have to wait etc. Also detects if power is getting too low. Don't think is very expensive.
A guy in Vic Park Perth, Brian from Car Stereo City, is full bottle on these things and does them every day. Give him a call and say Dave from Rycom gave you his name.