I thought it was the chimney, I thought it was the door opening but I updated all of those and yet the back of the oven still gets soaked after rain storms. The only thing I can think of is it’s actually soaking through the oven, but to do that it would have to soak through the mortar, insulation, and firebricks…which doesn’t seem probable. How else could it be getting so wet inside the oven?
It would seem more likely to me that it's getting in through the exposed firebricks in the front.then making it's way (pooling) to the back. I really doubt it's soaking through all the layers you mentioned.
Do you have a cover over the oven at all? I know the brink will absorb moisture, but there should not be “soaked” is there crack on your oven or a crack along where the brink or mortar meet your hearth? Can you post some pics to help us visualize whats going on?