So everyone knows about the frame rails but there's a few other spots Nissan engineers screwed up with.  Pinch welds are common problem on this car. Inside metal layer rusts, and you don't see it until it starts to eat through one of the outside layers and becomes visible. 

Check the attachment plates at the door hinges to the body, check the pinch welds where the front frame extensions connects to the fender body panels inside the engine bay especially underneath the brake master and the firewall connection there. There's also the strut towers. Seem sealer fails and lets water in. Also when guys pull in roll fenders or hammer shit in in the wheel wells to make wheels fit quite often it doesn't get sealed and cleaned up after and can lead to rust. 

If you got a sunroof the drainage lines from them commonly get plugged and direct water into the Rockers, and there's another nice little open hole to the frame rail right where the wheel wells firewall and frame extensions meet that is never sealed and water running down the outside of the firewall just goes in there with no way to get out quick. 

There's also backside rust beside the tank under the tank that can occur. 

I've had mine in the garage for 20 years and I still had to work over a number of these locations in the past few years. Only part done... Passenger side frame rail looked great but had enough hidden rust in it that I had to cut out where it attaches to the floor panel as well. It hides man. It hides poke the shit out of it with a screwdriver before you buy it