Perhaps you can find something to cap the inboard side of the crankarm to keep moisture out. But still, if the circuit board isn't held properly, that'll eventually be an issue.
If you haven't already done so, Garmin has a forum for all their sporting devices where their customers/users can help each other. Here is a link to the one for Rally pedals....
https://forums.garmin.com/sports-fit...ycling/f/rally
Maybe someone there has already had to deal with your issue.