Is this a stock spoiler? A few pictures would help. The stock spoiler is made up of two pre-formed pieces of plastic sandwiched together, probably with some type of adhesive. The seam, however, should be on the bottom. The fact that there's no gasket between the spoiler and trunk lid is irrelevant. If this is an aftermarket spoiler, then the seam may be facing up, which would allow water to get inside the cavity in between the two halves.
Here's an idea...take the spoiler off. put some plumbers putty around the holes where the bolts are, and submerge the thing in a swimming pool. Keep it in the same position as it would be on the car (right side up) while it's under the water. Look for bubbles.