Subwoofer Wont Work

I have a 2000 BMW 740i. I recently bought a sony xplod sub and amp. I went to a local audio store and had them wire and hook it up. 5 Min into using it it stopped, I opened the trunk and there was smoke coming from the box. Sub fried. I went back and told them to replace it which I did and the same result occured. So I go buy a new JBL amplifier, a new Kenwood sub and hook it all up and I start to smell a burning smell again (which I think is the coils burning). Does anybody know why this is happening? I have a built in amp in my car but idk if that has anything to do with it. Also maybe theres a slice in the wire and its touching metal? Any ideas?