I was searching the archives for something and came across Bob Feld's description of a rainwater leak in his 1996 C28. I had the same problem and traced it to the small "sump" in the cockpit behind the helm, where the rudder quadrant is found under a cover. Rain will inevitably get under the cover and into this sump area; it was filling the area and leaking down the hole drilled for helm instrument wires travelling down to the area under the cockpit sole. Luckily, after much searching I found a small drain that Catalina built into the aft of the sump, partially hidden by the quadrant. I took a heavy insulated wire and reamed out the drain hole, which had clogged with gunk. It solved my problem; any water that gets into the sump drains now, before it can back up into the wire run to the area below. Try this Bob if nothing else works; good luck.