I've not had to get to the back of my instrument cluster yet. It should be reachable from underneath though, there's a bit of room but you might need either thin hands or a university degree in contortionism.
You might be able to do it through the centre console - if you lift the tray above the stereo (lift the edge furthest from the windscreen with a smooth blade, a strong fingernail may do, or a plastic knife), then pull the thing backwards. Two screws will then be visible, undo these. The entire console area will then be able to pull towards the rear of the vehicle. You should then be able to reach inside towards the cluster and feel around the back.
There should be two electrical connectors on the back, just try to nudge them into place, I can't imagine why Nissan would have not used locking retainers on them but I've seen two people complain that the plugs were loose!
While you have that centre console out, add a black wire from one of the dash mount bolts underneath to the chassis of the stereo. There are 4 phillips (cross) head screws holding the stereo in, just put the wire under one of those. This should help the earth for the instrument cluster as well - just in case that was the problem.