I had a similar problem a while back on my '02 PF.  I have the
automatic climate control, so the parts were more expensive, but it was
the same general idea.  I got some help here and I got the Nissan
service bulletin for this problem from alldata.com.  The bulletin
specified the parts to replace and estimated the labor at 0.3 hours.  I
took the service bulletin to my dealer and told him that I wanted him
to do this service.  The service manager crossed out the 0.3 hours and
estimated it at 1.3 hours (an extra $90).  I asked why and he said that
the factory doesn't allow for "diagnostic" time.  I told him that I
didn't understand that because I wasn't asking for diagnosis.  I wanted
him to do the service that was in the bulletin.  He didn't back down.
I had him order the parts and schedule an appointment because I thought
I didn't have time to do it myself.

When the parts arrived, I showed up for my appointment.  The dealer
couldn't keep the appointment because the local mechanics (St. Louis)
were on strike.  I bought the parts and did the repair myself.  It took
me 20 minutes, or in other words, 0.3 hours.  I would expect that a
real mechanic (not me) could do it in even less time.

I think the diagnostic charge is often just a way for dealers to pad
their profits.  It's not the mechanics' fault, but if the dealer
charges an extra hour on 5 jobs a day, he pays the mechanic for 8 hours
and pockets the extra labor cost that never really happened anyway.