Hi mate, it will 99% be the seat connections that is causing the problem. The plug that you see under the passenger seat is normal mate. It's because your drivers seat has 2 switches and 2 motors for adjustment and the passenger seat only has one, hence no need for anything to be connected.
The light will not go off by simply fixing a contact problem on the under seat wiring. You need to have it re-set by a diagnostic specialist (they will be far far cheaper than Nissan!).
You should remove each connection one by one and reconnect making sure they are secure. Once complete make the call to a local specialist or mobile diagnostic guy in the back of your local autotrader to have fault code removed. Will cost approx £35-64ish.
Let us know how you get on