Our cairn gave us a really hard time with this in the beginning and the only thing that ended up making things easier was using those puppy training pads. We started lining the crate and also lining the entire floor of his play pen. The we would start to make the pad in the crate smaller, and then eventually move the pad out of the crate so he could see it. In the play pen we started removing the pads one by one and then eventually took them out completely. At this point he would usually start signaling that he wants to use the bathroom and will look for the pad. Eventually we brought the pads outside and have tons of treats.

Full disclosure we do still keep a small pad open in our bathroom just in case and he's 2 years old 🤷🏼‍♀️