Jeff, I believe what you have there is the air dam that mounts up under the front bumper. My dealer neglected to install mine, too, and I found it in the trunk along with a package of mounting hardware. The air dam is indeed just a little shorter than the width of the trunk (longer than the 18" that you remember) and about 2"+ wide. The dealer installed it in just a few minutes---guess they hadn't sold too many Prii at that point (April '02). Glad I hadn't paid extra for "dealer prep"!