i recently hooked up a subwoofer that draws 12v directly from the battery. the 12v cable has a 30amp fuse right near the start of the cable near the battery. When i first put a fuse into the amp, i started my car and it immediately died after about 3 seconds of putting it into gear. When i found out the problem was my ignition fuse, I replaced it and turned the car on again. I made it about 30 feet when the fuse blew again. Ive been through about 5 fuses so far and it keeps blowing. What ive discovered is that when i take the fuse out of the subwoofer cable, i stop blowing my ignition fuse. What could be causing this?

 

Ground the sub also at the battery just to see if it still does it. MAybe when you switch the ignition on, 12v is back feeding to the sub somehow through that circuit and blowing the fuse.