Not only is it a fire hazzard, but depending on what kind of car you have, some electrical systems run through the car's ECM for multiplex control (i.e. the same wire carries ground one second, then +12 vdc, then -12 vdc, then data or signal -- all on the same line) so you may run the risk of blowing your car's ECM, which is only anywhere from 300.00 to 2500.00.