Yep, as said, anywhere on the body. On Escorts (probably similar to the Mondeo) if you take the footwell covers off there's plenty of holes where you can put a screw through with the earth cable with a ring connector on it and screw it to the body. Just make sure you don't screw into any other wiring and that the connection is as tight as it can be and Roberts your relation.

In fact you might find an earth point already set up that you can add an additional cable to, I tend not to do that though, because I don't want to risk upsetting stuff that's already there.