Led light bar overheat

I was trying to install two led light bar under the dash inside my car, one on each side. Each led is 12v, 6w as it said on the package! I use the fuse tap or add-a-circuit method as it called to power the led. I followed all the proper instructions, positive to the cigarette lighter fuse and the negative to ground! The led lighted up when I turned the key to ACC (engine not start yet), everything was fine however when I started the engine, both led light bar heated up in a few seconds. The heat even melt the double side tape at the back and start to smoke a bit! They were so hot can't even touch! I know led do generate some heat but not that much! Now my question is why it happened only when the engine start? Too much current? Bad led light bar or do I need a resistor? By the way I did the same thing in my previous car for five years, nothing went wrong! Please, any clue?
Thanks