The Car Mouse isn't living in the car at the moment. We know that because there's no sign of renewed mouse activity in our now pepperminty scented car (see replies to first post), but also because there's definite proof he/she is still living in our garage. I had some marzipan left from icing the Christmas cake and hubby decided to put it in the garage as a mousie Christmas gift on Christmas eve.
On Christmas day he checked and... the marzipan ball has been dragged to the back wall of the garage and partially devoured.
I've been doing more research and discovered that the name "field mouse" is actually a term used to describe the Wood Mouse. Wood mousies live wild, but will move into houses in winter. They're less destructive than house mice and never stay beyond spring.
For those who aren't rodent aware, below is a photo of both the House Mouse (left) and Wood/Field Mouse (right). :-)