Nissan refers to that gauge as the Trip Computer in the FSM, that might help if you're searching for solutions on your own.
My first suggestion would be to check the fuse, it's a 10A fuse located at #19 on the fuse block. You said you traced the wires and made sure they weren't broken, did you make sure that the harness was plugged into the Unifed Meter & AC Amp? It's behind/underneath the radio. The harness can get disconnected when upgrading from the stock radio.
After that I'd suggest that the gauge may actually be faulty, though I've never heard of one being completely dead - exactly why my first suggestion is to check the fuse.
Welcome to NICO!