mini-box the company isn't very good with customer service, i have a thread here you might want to check out.
i wouldn't rely on them.... try an RMA from the company you bought it from if it's still under warranty.
also, isn't that what the fuse is for? to protect the power circuit?
first thing i'd try is taking out the fuse, plugging ignition/ground/12v everything in, pulling the fuse, and checking for power there..... if there's power there, then well i dunno, if there's not, then you can investigate from the wiring outside the PSU to the fuse itself.