Chris,
I had the same problem after having my spoiler fitted, only came to light after a sustained period of wet weather.
Probable cause, as you suspect, a leak somewhere from your spoiler, like one of the nuts/bolts or the wires that feed the brake light on the spoiler.
HOW TO IDENTIFY OFFENDING NUT/BOLT
HOW TO CURE PROBLEM
Or as I did, remove the nut from inside of boot lid, slip on large rubber washer, pop nut back on and tighten. Then cover with clear outdoor all weather sealant. Also taped up the wires that come from the inside of boot lid for the light on the spoiler and then applied the sealant to that area too.
Now, no more leaks.
And this little leak does lead to a little swimming pool under the spare tyre if left unchecked.