This site will tell you where to look
GM - Cars - 1985 and newer the codes are in the trunk. They will be either under the rug, on top of the spare tire, or underneath the trunk lid. The codes will be on a white sticker about 3" x 4". The paint codes will be all the way on the bottom of the sticker. They are called "WA" numbers on a GM. They will consist of either 4 numbers or 3 numbers and a letter. (ex. wa8779, wa279e). The letters "WA" do not always appear, the numbers may be prefaced by bc/cc, L, or U, for upper and lower colors.
I never knew that!