Override mirage response in Acceptance tests?

In our Ember app [v.2.14], we're using mirage to mock our APIs and have provided the direct response for our model 'car'

mirage/config.js:

 this.get('/cars', () => {
 return {
  "data": {
    "type": "car",
    "id": "1",
    "attributes": {
     "exists" : true
    }
   }
 }

In our acceptance tests, I want to override this.
I understand that using factories and fixtures we can feed the mirage store which CAN be overridden in the tests using server.create().
It doesn't work though when I am providing the direct response as above. Any clues if it can be done?