Ok first the whine. Take the ground wire from your amp and make the shortest connection to the car's sheet metal that you can. Don't just go find a bolt somewhere and tie the wire under it. Go out and get a self tapping screw and a crimp on ring terminal. Sand the paint off the area where you are going to ground the amp and then drill a hole for the screw and secure it all. Be careful as to what is on the other side of that hole!!!

As for the shutting down of the amp. You obviously have too much of a load on it. I am betting that you have the + from both speakers (front and rear) to the + terminal of the amp and the same setup for the - terminals (parallel wiring). This causes a lower resistance and higher current requirement for the amp. You have two choices. Either go + from amp to + from one speaker and then - from that speaker to + from other speaker and then from - of that speaker to - of the amp (series wiring), or only connect one set of speakers.