Radiator Fan won't switch off- replaced relay still not switching off

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I have a 1991 VW Golf Cabriolet and my radiator fan is not switching off. It's drained the battery. I have replaced the relay but still not switching off. I left it a couple of days, jump started it and topped up the water, it ran fine for about 20 mins then the fan kicked in again- it's really loud like it's on overdrive or something and the only way to get it stopped it to take the relay out every time I stop the car. Any ideas?