The relay installation design unfortunately is a problem. The bottom of the relay is located with a full circle locating fitting, which while probably not watertight, certainly retards drainage. But the top is located by two plates on opposite sides welded to the top of the chassis, leaving a significant gap on the other two sides, allowing water and dust to fill the annulus between the relay and the tube.

It is easy to see how this design could have been improved, for example by some sort of a seal or baffle at the top - just a turned land on the relay with pressed ring similar to the one on the bottom welded or bolted in the top of the tube, for example. Even if this rusted together it would still be easy to free.

But let's face it - the original design, which was unchanged for nearly forty years, remained troublefree for the expected life of the vehicle (probably no more than 100,000miles/10 years).