That's a different subject matter. But how is Ford supposed to be able to guarantee the functioning of a car that they didn't make? Since you made it yourself by making changes? It makes sense that they would challenge such warranty claims all the way to the court.

A lot of things on cars are welded or otherwise very difficult for the customer to modify, and I don't think any court would have a big problem with that.

They have to prove that the changes you made is what caused the failure. If they can't and/or the changes are unrelated to the warranty claim, they have to honor it.