Trying to identify a Breaker in my electric panel...

The issue is this...lol

When I connect my christmas lights up through a very old extension cord to my outside sockets, and if it rains, the breaker being discussed trips quite a bit.
The diesel truck is also plugged in to the same outlet through the other extension cord that is just as old. In both extension cords, there are three new ends and one original.

At first, I suspected the breaker. But the breaker never trips the rest of the year. And it pretty much happens only when it rains.