I have installed 2 new speaker units in the back of my car using high quality connectors and wires but am having trouble with the sound. I have matched the speakers to the stereo output at 50W speakers to 4 x 50W out on stereo. I have also checked to make sure the plus and minus are correct way round but whenever i have my stereo on the sound is fine for about 10 seconds then goes very quiet and very crackly and am unsure why :S any ideas please?