Is the aircraft hangared or out in the open? If its out in the open, does it have a cover over it? What condition is the cover in?
Our group aircraft was parked in the open air, with a canvas cover over the canopy. After only one winter, the interior was damp as the canvas absorbed rain. We bought a new heavy duty canopy and cowling cover and have had no trouble ever since. We know that our canopy isn't water-tight, in drizzle we get water running up the windscreen and drips coming through the canopy bow, but even after heavy rain, when we remove the cover, the interior is still dry.