91's have to have fan relays. besides the stock clutch fan they have a secondary electric fan that sits smack up against the radiator on the engine side for the air conditioning . Unless of course you have a car that came with no air. All cars with air had this fan and therefore have a relay to turn it on and off. 91 and up they were setup to come on automatically if the car was running under 35 mph and was getting hot.
So given this if you have air you are quite possibly missing your electric fan relay.
As far as what the guy at Nissan said . Nissan's parts computers are far from perfect. The part number for your car may have been superseded and the parts guy was looking at the newer number.
Also Nissan has some inaccuracies in their parts computer files . I have run into this on more than one occasion .
Example would be listing a part that was availible for our cars from the cars inception but later on in the pages say that part was only offered from 7/90 forward i.e. 91 model year. go figure.
This has been a source of frustration for several of the fact freaks here in socal. Me being one of them. <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/tounge.gif" border="0" valign="absmiddle" alt=':p'>
Remember parts computers and computers in general are only as smart as the people who program the software.
I have learned the hard way I have to be skeptical some times . Man it sucks being jaded <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/hehe.gif" border="0" valign="absmiddle" alt=':hehe:'>