If you drilled through the firewall thats a common grounding point where it will cut. Unplug everything from the amp, and put a volt meter to the amp end of the power wire. Use the black wire on the volt meter to ground that out, and put the red on the amp wire (maybe that overkill, u probubly know that). Have a friend put a fuse in and watch the meter. If the fuse pops your wire is grounding out somewhere, if it dosent pop and the volt meter says 12V your wire is ok, and the problem is in the amp or your ground is bad.
One time I got ahead of myself and spent 2 days trying to figure out why I was blowing fuses right away. Come to find out I had power and ground swapped on the amp. I felt like an idiot for blowing $20 worth of fuses.