For #1, if you don't mind spending a little extra cash you should
still be able to get the vents from the parts department at your
Nissan dealer.  I replaced one in my '95 Pathfinder a short while back
and it only took a few days to get in.  It was in the $40 range.

For #2, your dash *may* have everything behind it that you need to
install one from a donor vehicle.  I'd pull the instrument panel out
and check it out.  If you don't want to risk damaging yours, practice
at the salvage yard.

For #3, Alldata (www.alldata.com) use to have a CD they put out you
could buy which was a scan of a lot of the factory manual sections.  I
think most of their stuff is online only now.  Check there.  Short of
buying the factory manual from some place like eBay (like I did mine)
or from the publisher.  Here is their contact information that was
valid last time I used them:

Dyment Distribution Services
20770 Westwood Road
Strongsville, OH 44136
Fax: (216)572-0815
Phone: 1-800-247-5321

Enjoy,

Wil