One place that 123's always rust out is in the wells where the hood hinges live when the hood is closed. They get trash in them and the drain holes clog up. After they rust out the rain water runs under the dash into the front floor and then runs under the mats to the back and remains there. Few 123's are not rusted out here.

I fixed mine with roofers asphalt and fiberglass cloth. It's not elegant, but it holds up and keeps the water out.

Richard