Why does my car amp go in protect mode?

I have a 200 watt amp in my car, and everything worked fine. Then I upgraded to a 1000 watt amp, and it goes in protect mode only when I connect the RCA cables to it. Then I disconnected the RCA cables and the amp turns on. When I get the RCA cable and put the tip of it into the line input I can hear sound, but when I plug the RCA cables in all the way the amp goes back in protect mode. What could be the problem... Thanks in advance :-)