In my area, the biggest structural area to rust is the box section, extending forward from the fire wall to the headlight bucket area, under the front fenders. The first thing you'll notice is rust appears visible from the engine compartment, but the real damage happens underneath. I've seen this box section rust completely off on the top side, making attachment of a new fender all but impossible without a proper fix.
Of course no one has to mention low areas like the floors or the spare tire well in the trunk.
Another area of rust that I've noticed lately is the bottom side of the hood, near the front where water collects and just sits, same goes for the similar area on the trunk lid. Buy the time you see this damage, it's way to late as it rusts from the inside out. The bottoms of the doors are prone to this as well, and they are all areas that are difficult to repair.
The fuse box is often a source of problems - if the car has ever gotten wet. As Nissan realized it was a mistake to put the fuse box outside (rectified in every model that followed the PL510) - well a weathered fuse box may have issues. A good cleaning on an unknown car is a very good idea, as corroded contacts will cause heat, and melt the plastic fuse box without ever blowing a fuse. And this is a shame, as fuse boxes are getting hard to find.
Well, 510's have been notoriously owned by cheap people in the past, therefore people who are very unqualified just love to hack at wiring, believing they know better than the engineers at Nissan - what a group of freaks these home jobs are. So having broken the ice, first you need to look at the back of the ignition switch, and see how many add-ons the previous owners have tried - usually unsuccessfully - to hook up, and probably left the failed remains behind, still live. Remove anything that isn't factory and save your self a head ache!
The next area for the usual hack jobs is the "professional" stereo install guy who cuts any wire near the center console looking for one that has the type of power signal he desires - and all the cut wires usually get twisted back together with a wrap of black electrical tape if they are lucky! If you car ever had an alarm system installed in the 80’s of 90’s – you may consider a new wiring harness all together. I have seen too many “professionally” installed systems that have messed with every system in the car – I send you my deepest condolences if this is your acquired product!
The next area of electrical gremlins is often the headlight circuit. Here, as is often the case, the previous owner(s) have usually cut and spliced wires trying to bypass the head light relay that simply wore out. So instead of buying a new one, they rewire the car for what ever they can make work - no matter how scary or how un-fused or unsafe it may be.
A little input for tonight, but these are a few areas that come to mind.
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because the opposite never works.