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It depends.  The 92-97's basically use the same filter.  I pulled the K&N 33-2086 out of my 92 STS and installed it in my 96 STS and am happy with the fit.  

UPDATE:  after some consideration, I have also sealed the sides of the upper airbox (which contains the filter) with duct tape.  This should ensure that no amount of "flexing" of the airbox could cause air to enter the airbox around the filter seal.

Others have other comments:

In August Jay noted:
Jay>"I checked with my tech guy at K&N about the STS filter. They list
Jay>one for the '96 Eldo, but not the STS. I thought the front of
Jay>these two were the same and the Eldo filter should fit the STS. 
Jay>The K&N tech guy said that the STS had a "crappy" air box design,
Jay>and that the filter would fit, but would not seal completely."  Jay

In July John said:
John>I have a 1997 Seville STS.  And I put on a K&N filer part
John>number33-2057.  The filter tad to be modified by cutting a thin
John>strip of rubber off the top edge on the filter.  I used a razor
John>blade and the whole modification took less then 10 min.  Hope this
John>helps.  Good luck...

See web2.airmail.net/nunnally/sts/knfilterpns.htm for a listing of filter p/n by type of caddy and year.

I suppose if you want to be extra careful you should get the 33-2057 and trim to fit, which should ensure that there is no problem sealing.  

Good luck,
Bruce  
96 STS