a 4 ga wire is too small for more then just the 1200 to begin with. It's no wonder that you are breaking fuses. The largest I would go on that run of wire is about a 100A fuse...anything past that, and you are pushing the capacity of the wire. I would consider running a second line of 4 or 8 ga to the second amp (depending on it's size) and fusing both lines appropriately.

There are plenty of good grounding areas in the trunk, you just have to make sure that you keep them short, sand down to bare metal (not primer) and make sure it's on there tight.