Look under your throttle body. Look for the fuel rails. You should see a screw-on cap on it. Here's a pic of mine, with the upper intake off... It is on the end of the fuel rail near where the rail connects to the fuel line. 
1999 Mystique LS V6
Alpine CDA-9807, Rear deck sub, Opt. TB, K&N RU-3530/MAF adaptor, custom heatshield, FSVT 17's, SCT 4-program chip, Eibach's/ST200's, Hella Fogs...
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