There have been towbars fitted to ST's before, and I have seen a picture before of one pulling a trailer. However, it's not designed to. The problem is that the bumper is just too low and because it's been lowered with stiff suspension, when you pull a caravan or a trailer then this makes the back end go too low (it could pull up the front) and it's considered dangerous.
When you look in the Fiesta manual it lists the tow weight of an ST as not applicable, and this is because Ford won't supply or fit one, although you would be able to get someone to custom make you one. If anything went wrong however, then the warranty would be invalid.
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