Ive seen a similar problem in the Mk1 fiestas ive owned, and im afraid there's only one cure ive come up with to date...
its not for the faint hearted !
You'll need to strip the door and remove any flammable items from it (including any rubbers in the immediate area of the crack.) Clean the crack back to bare metal, then using a 3mm drill bit...drill a hole right at the end of the crack. Cracks spread because there is a point of pressure at the end of them, so by drilling this out, the pressure is relieved and the crack stops. To make a proper repair, you then have to carefully weld the hole and crack back up with a mig. Remember to weld in short bursts and use a wet rag to cool the area after each weld, or distortion will set in.
Hope this helps

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