Hi, i dont know if the petrols are same as diesels but it could be the resistor which is under dash. to get access, pull out glove box and you will see it on the right screwed to side of dash.Pull out connector and unscrew.I think it is orange or red (but it could be blue lol).The motor is down there too.
as stated earlier resistor packs blown! the fan speed is controlled by dropping resistors into the feed line to the motor, the slower the speed the higher the resistor, hence when they all fail the fan only runs on full speed as this is a direct feed via the switch, its a simple job to replace it, the only tools you will need will be a Torx bit or allen key to undo the retaining bolt holding the resistor pack to the heater unit
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